[llvm] [NFC][AMDGPU] Add tests for an fmul split from its fadd/fsub user (PR #213303)

[Matt Arsenault via llvm-commits]

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+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 6
+; RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx803 < %s | FileCheck -check-prefix=GFX8 %s
+; RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 < %s | FileCheck -check-prefix=GFX9 %s
+; RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx1030 < %s | FileCheck -check-prefix=GFX10 %s
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arsenm wrote:

```suggestion
; RUN: llc -mtriple=amdgpu8.03-amd-amdhsa < %s | FileCheck -check-prefix=GFX8 %s
; RUN: llc -mtriple=amdgpu9.00-amd-amdhsa < %s | FileCheck -check-prefix=GFX9 %s
; RUN: llc -mtriple=amdgpu10.30-amd-amdhsa < %s | FileCheck -check-prefix=GFX10 %s
```

https://github.com/llvm/llvm-project/pull/213303