[llvm] [LTO] Avoid assert fail on failed pass plugin load (PR #96691)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-aarch64-darwin` running on `doug-worker-4` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/190/builds/744

Here is the relevant piece of the build log for the reference:
```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM :: Feature/load_plugin_error.ll' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 4: not /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/opt < /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/Feature/load_plugin_error.ll -load-pass-plugin=/Users/buildbot/buildbot-root/aarch64-darwin/build/test/Feature/Output/load_plugin_error.ll.tmp/nonexistent.so -disable-output 2>&1 | /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/FileCheck /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/Feature/load_plugin_error.ll
+ not /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/opt -load-pass-plugin=/Users/buildbot/buildbot-root/aarch64-darwin/build/test/Feature/Output/load_plugin_error.ll.tmp/nonexistent.so -disable-output
+ /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/FileCheck /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/Feature/load_plugin_error.ll
RUN: at line 6: /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/opt /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/Feature/load_plugin_error.ll -o /Users/buildbot/buildbot-root/aarch64-darwin/build/test/Feature/Output/load_plugin_error.ll.tmp.o
+ /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/opt /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/Feature/load_plugin_error.ll -o /Users/buildbot/buildbot-root/aarch64-darwin/build/test/Feature/Output/load_plugin_error.ll.tmp.o
/Users/buildbot/buildbot-root/aarch64-darwin/build/bin/opt: WARNING: failed to create target machine for 'x86_64-unknown-linux-gnu': unable to get target for 'x86_64-unknown-linux-gnu', see --version and --triple.
RUN: at line 7: not /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/llvm-lto2 run -load-pass-plugin=/Users/buildbot/buildbot-root/aarch64-darwin/build/test/Feature/Output/load_plugin_error.ll.tmp/nonexistent.so /Users/buildbot/buildbot-root/aarch64-darwin/build/test/Feature/Output/load_plugin_error.ll.tmp.o -o /Users/buildbot/buildbot-root/aarch64-darwin/build/test/Feature/Output/load_plugin_error.ll.tmp      -r /Users/buildbot/buildbot-root/aarch64-darwin/build/test/Feature/Output/load_plugin_error.ll.tmp.o,test 2>&1 |    /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/FileCheck /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/Feature/load_plugin_error.ll
+ not /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/llvm-lto2 run -load-pass-plugin=/Users/buildbot/buildbot-root/aarch64-darwin/build/test/Feature/Output/load_plugin_error.ll.tmp/nonexistent.so /Users/buildbot/buildbot-root/aarch64-darwin/build/test/Feature/Output/load_plugin_error.ll.tmp.o -o /Users/buildbot/buildbot-root/aarch64-darwin/build/test/Feature/Output/load_plugin_error.ll.tmp -r /Users/buildbot/buildbot-root/aarch64-darwin/build/test/Feature/Output/load_plugin_error.ll.tmp.o,test
+ /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/FileCheck /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/Feature/load_plugin_error.ll
/Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/Feature/load_plugin_error.ll:11:10: error: CHECK: expected string not found in input
; CHECK: Could not load library {{.*}}nonexistent.so
         ^
<stdin>:1:1: note: scanning from here
llvm-lto2: LTO::run failed: No available targets are compatible with triple "x86_64-unknown-linux-gnu"
^
<stdin>:1:83: note: possible intended match here
llvm-lto2: LTO::run failed: No available targets are compatible with triple "x86_64-unknown-linux-gnu"
                                                                                  ^

Input file: <stdin>
Check file: /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/Feature/load_plugin_error.ll

-dump-input=help explains the following input dump.

Input was:
<<<<<<
            1: llvm-lto2: LTO::run failed: No available targets are compatible with triple "x86_64-unknown-linux-gnu" 
check:11'0     X~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ error: no match found
check:11'1                                                                                       ?                     possible intended match
>>>>>>

--

********************


```

https://github.com/llvm/llvm-project/pull/96691