[llvm] AMDGPU: Implement i1 to bfloat conversion (PR #130831)

[Matt Arsenault 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 < %s -mtriple=amdgcn -mcpu=gfx942 | FileCheck %s -check-prefix=GFX942
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arsenm wrote:

Should also have tests on other targets, particularly the ones without legal bf16 

https://github.com/llvm/llvm-project/pull/130831