[llvm] [AMDGPU] Add assembler check for GFX1250 unclaused vmem workaround (PR #209517)

[Shilei Tian via llvm-commits]

shiltian wrote:

> 1. The warning is not actionable unless you already know exactly what sequence it is checking for. I guess the only solution to that is to make the warning message more verbose.

Agreed. If we want, we could add some words in the AMDGPU usage document.

> So perhaps we could just check that the first two instructions are V_NOP and any VMEM, in either order?

Sounds reasonable to me.

https://github.com/llvm/llvm-project/pull/209517