[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 `llvm-nvptx64-nvidia-win` running on `as-builder-8` while building `llvm` at step 7 "test-build-unified-tree-check-llvm".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/155/builds/1901

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

```
Step 7 (test-build-unified-tree-check-llvm) failure: test (failure)
******************** TEST 'LLVM :: CodeGen/MIR/X86/fake-use-tailcall.mir' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 25
c:\buildbot\as-builder-8\llvm-nvptx64-nvidia-win\build\bin\llc.exe -run-pass=codegenprepare -o - C:\buildbot\as-builder-8\llvm-nvptx64-nvidia-win\llvm-project\llvm\test\CodeGen\MIR\X86\fake-use-tailcall.mir | c:\buildbot\as-builder-8\llvm-nvptx64-nvidia-win\build\bin\filecheck.exe C:\buildbot\as-builder-8\llvm-nvptx64-nvidia-win\llvm-project\llvm\test\CodeGen\MIR\X86\fake-use-tailcall.mir
# executed command: 'c:\buildbot\as-builder-8\llvm-nvptx64-nvidia-win\build\bin\llc.exe' -run-pass=codegenprepare -o - 'C:\buildbot\as-builder-8\llvm-nvptx64-nvidia-win\llvm-project\llvm\test\CodeGen\MIR\X86\fake-use-tailcall.mir'
# executed command: 'c:\buildbot\as-builder-8\llvm-nvptx64-nvidia-win\build\bin\filecheck.exe' 'C:\buildbot\as-builder-8\llvm-nvptx64-nvidia-win\llvm-project\llvm\test\CodeGen\MIR\X86\fake-use-tailcall.mir'
# .---command stderr------------
# | C:\buildbot\as-builder-8\llvm-nvptx64-nvidia-win\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: C:\buildbot\as-builder-8\llvm-nvptx64-nvidia-win\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