[llvm] AMDGPU: Fix selection failure on fast vector rootn (PR #185035)

[Steffen Larsen via llvm-commits]

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+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 6
+; RUN: opt -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 -passes=amdgpu-simplifylib,instcombine < %s | llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 | FileCheck %s
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steffenlarsen wrote:

Minor personal preference, but this would benefit from a small comment stating the reason why a codegen test is needed. The git-history papertrail should lead back to this, but if someone comes along and breaks the expectations here it would help to be able to quickly identify the important parts of this test, for either the breaker or the reviewers.

https://github.com/llvm/llvm-project/pull/185035