[llvm] [ExtendLifetimes] Implement llvm.fake.use to extend variable lifetimes (PR #86149)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-s390x-linux-lnt` running on `systemz-1` while building `llvm` at step 7 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/136/builds/770

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 7 (ninja check 1) failure: stage 1 checked (failure)
******************** TEST 'LLVM :: CodeGen/X86/fake-use-zero-length.ll' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 1: /home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/stage1/bin/llc < /home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/llvm/llvm/test/CodeGen/X86/fake-use-zero-length.ll -stop-after=finalize-isel | /home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/stage1/bin/FileCheck /home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/llvm/llvm/test/CodeGen/X86/fake-use-zero-length.ll --implicit-check-not=FAKE_USE
+ /home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/stage1/bin/FileCheck /home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/llvm/llvm/test/CodeGen/X86/fake-use-zero-length.ll --implicit-check-not=FAKE_USE
+ /home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/stage1/bin/llc -stop-after=finalize-isel
/home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/llvm/llvm/test/CodeGen/X86/fake-use-zero-length.ll:14:10: error: CHECK: expected string not found in input
; CHECK: %[[IN_VREG:[a-zA-Z0-9]+]]:gr32 = COPY $[[IN_REG]]
         ^
<stdin>:82:15: note: scanning from here
 liveins: $r2l
              ^
<stdin>:82:15: note: with "IN_REG" equal to "r2l"
 liveins: $r2l
              ^
<stdin>:84:2: note: possible intended match here
 %0:gr32bit = COPY $r2l
 ^

Input file: <stdin>
Check file: /home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/llvm/llvm/test/CodeGen/X86/fake-use-zero-length.ll

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

Input was:
<<<<<<
            .
            .
            .
           77: debugValueSubstitutions: [] 
           78: constants: [] 
           79: machineFunctionInfo: {} 
           80: body: | 
           81:  bb.0.entry: 
           82:  liveins: $r2l 
check:14'0                   X error: no match found
check:14'1                     with "IN_REG" equal to "r2l"
           83:   
check:14'0     ~~
           84:  %0:gr32bit = COPY $r2l 
check:14'0     ~~~~~~~~~~~~~~~~~~~~~~~~
check:14'2      ?                       possible intended match
           85:  CallBRASL @bar, csr_systemz_elf, implicit-def dead $r14d, implicit-def dead $cc, implicit $fpc 
check:14'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           86:  $r2l = COPY %0 
check:14'0     ~~~~~~~~~~~~~~~~
           87:  CallBRASL @baz, $r2l, csr_systemz_elf, implicit-def dead $r14d, implicit-def dead $cc, implicit $fpc 
check:14'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
...

```

</details>

https://github.com/llvm/llvm-project/pull/86149