[llvm] AMDGPU: Use real copysign in fast pow (PR #97152)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-x86_64-debian-dylib` running on `gribozavr4` while building `llvm` at step 7 "test-build-unified-tree-check-llvm".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/60/builds/1438

Here is the relevant piece of the build log for the reference:
```
Step 7 (test-build-unified-tree-check-llvm) failure: test (failure)
******************** TEST 'LLVM :: CodeGen/AMDGPU/amdgpu-simplify-libcall-pow-codegen.ll' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 2: /b/1/llvm-x86_64-debian-dylib/build/bin/opt -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 -passes=amdgpu-simplifylib,instcombine -amdgpu-prelink < /b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/test/CodeGen/AMDGPU/amdgpu-simplify-libcall-pow-codegen.ll | /b/1/llvm-x86_64-debian-dylib/build/bin/llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 -amdgpu-prelink | /b/1/llvm-x86_64-debian-dylib/build/bin/FileCheck /b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/test/CodeGen/AMDGPU/amdgpu-simplify-libcall-pow-codegen.ll
+ /b/1/llvm-x86_64-debian-dylib/build/bin/FileCheck /b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/test/CodeGen/AMDGPU/amdgpu-simplify-libcall-pow-codegen.ll
+ /b/1/llvm-x86_64-debian-dylib/build/bin/opt -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 -passes=amdgpu-simplifylib,instcombine -amdgpu-prelink
+ /b/1/llvm-x86_64-debian-dylib/build/bin/llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 -amdgpu-prelink
/b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/test/CodeGen/AMDGPU/amdgpu-simplify-libcall-pow-codegen.ll:62:15: error: CHECK-NEXT: is not on the line after the previous match
; CHECK-NEXT: v_cvt_f16_f32_e32 v1, v1
              ^
<stdin>:76:2: note: 'next' match was here
 v_cvt_f16_f32_e32 v1, v1
 ^
<stdin>:74:24: note: previous match ended here
 v_log_f16_e64 v3, |v0|
                       ^
<stdin>:75:1: note: non-matching line after previous match is here
 s_movk_i32 s4, 0x7fff
^
/b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/test/CodeGen/AMDGPU/amdgpu-simplify-libcall-pow-codegen.ll:105:15: error: CHECK-NEXT: expected string not found in input
; CHECK-NEXT: v_and_or_b32 v0, v1, v0, v2
              ^
<stdin>:126:26: note: scanning from here
 v_mul_f32_e32 v2, v2, v3
                         ^
<stdin>:127:2: note: possible intended match here
 v_and_b32_e32 v0, v1, v0
 ^
/b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/test/CodeGen/AMDGPU/amdgpu-simplify-libcall-pow-codegen.ll:121:15: error: CHECK-NEXT: expected string not found in input
; CHECK-NEXT: v_writelane_b32 v43, s16, 14
              ^
<stdin>:152:26: note: scanning from here
 s_mov_b64 exec, s[18:19]
                         ^
<stdin>:153:2: note: possible intended match here
 v_writelane_b32 v43, s16, 15
 ^
/b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/test/CodeGen/AMDGPU/amdgpu-simplify-libcall-pow-codegen.ll:357:15: error: CHECK-NEXT: is not on the line after the previous match
; CHECK-NEXT: v_mul_f16_e32 v2, v3, v2
              ^
<stdin>:420:2: note: 'next' match was here
 v_mul_f16_e32 v2, v3, v2
 ^
<stdin>:418:26: note: previous match ended here
 v_cvt_f16_f32_e32 v2, v2
                         ^
<stdin>:419:1: note: non-matching line after previous match is here
 s_movk_i32 s4, 0x7fff
...

```

https://github.com/llvm/llvm-project/pull/97152