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

[Jay Foad via llvm-commits]

jayfoad 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.

How about:
```
warning: kernel 'foo' does not begin with the required prologue sequence: GLOBAL_WB followed by V_NOP
```

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

After some offline discussion I think relaxing the check is not helpful. We want consistency between various tools that check whether the prologue sequence is present, so it's best to standardise on the exact sequence of two instructions / four dwords.

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