[llvm] AMDGPU: Make v2f64 -> v2f16 conversion Legal only when unsafe fast math is set (PR #134738)

[Changpeng Fang via llvm-commits]

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+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
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changpeng wrote:

> Test belongs in the existing fptrunc.f16.ll test

The existing fptrunc.f16.ll only checks when unsafe fpmath is set.  This patch actually fixes the codegen when unsafe fpmath is not set for v2f16 only. Maybe we should not include scalar and wider in this file?  

https://github.com/llvm/llvm-project/pull/134738