[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-ppc64-aix` running on `aix-ppc64` while building `llvm` at step 3 "clean-build-dir".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/64/builds/820

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

```
Step 3 (clean-build-dir) failure: Delete failed. (failure) (timed out)
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM :: CodeGen/MIR/X86/fake-use-tailcall.mir' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 25: /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/bin/llc -run-pass=codegenprepare -o - /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/test/CodeGen/MIR/X86/fake-use-tailcall.mir | /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/bin/FileCheck /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/test/CodeGen/MIR/X86/fake-use-tailcall.mir
+ /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/bin/llc -run-pass=codegenprepare -o - /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/test/CodeGen/MIR/X86/fake-use-tailcall.mir
+ /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/bin/FileCheck /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/test/CodeGen/MIR/X86/fake-use-tailcall.mir
/home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/test/CodeGen/MIR/X86/fake-use-tailcall.mir:29:15: error: CHECK-NEXT: is not on the line after the previous match
# CHECK-NEXT: call{{.*}}fake.use(i32 %i)
              ^
<stdin>:23:9: note: 'next' match was here
 notail call void (...) @llvm.fake.use(i32 %i)
        ^
<stdin>:12:10: note: previous match ended here
 if.then: ; preds = %entry
         ^
<stdin>:13:1: note: non-matching line after previous match is here
 %call = tail call i32 (...) @f0()
^

Input file: <stdin>
Check file: /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/test/CodeGen/MIR/X86/fake-use-tailcall.mir

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

Input was:
<<<<<<
         .
         .
         .
        18:  br label %if.end 
        19:   
        20:  if.end: ; preds = %if.else, %if.then 
        21:  %temp.0 = phi i32 [ %call, %if.then ], [ %call1, %if.else ] 
        22:  notail call void (...) @llvm.fake.use(i32 %temp.0) 
        23:  notail call void (...) @llvm.fake.use(i32 %i) 
next:29             !~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  error: match on wrong line
        24:  ret i32 %temp.0 
        25:  } 
        26:   
        27:  declare i32 @f0(...) local_unnamed_addr 
        28:   
         .
         .
         .
>>>>>>

--

...

```

</details>

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