[llvm] [NFC][AMDGPU] Add test for unfold-masked-merge-scalar-variablemask.ll (PR #140093)

[Shilei Tian via llvm-commits]

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+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc -mtriple=amdgcn -mcpu=gfx1100 < %s | FileCheck --check-prefix=GCN %s
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shiltian wrote:

nit: use full triple name, aka `amdgcn-amd-amdhsa`, here

https://github.com/llvm/llvm-project/pull/140093