[llvm] [AMDGPU] Ensure non-reserved CSR spilled regs are live-in (PR #146427)

[via llvm-commits]

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+; Just ensure that llc -O1 does not error out
+; RUN: llc -O1 -mtriple=amdgcn-amd-amdhsa -mcpu=gfx908 -verify-machineinstrs %s -o - &>/dev/null
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macurtis-amd wrote:

> Should drop the -verify-machineinstrs, check the output, and still try to make a synthetic test

@arsenm I decided to create an mir test instead, which reduced fairly nicely IMO and is more targeted to the (interdependent) codegen passes conspiring to produce the failure. Is this ok?

https://github.com/llvm/llvm-project/pull/146427