[llvm] [AMDGPU] Add missing comma between export target and first export data (PR #181641)

[Mariusz Sikora via llvm-commits]

mariusz-sikora-at-amd wrote:

> > > Does the assembler still support the old form without the comma, and are there tests for that?
> > 
> > 
> > Yes, it supports both forms.
> 
> Just curious: how does that work?

I think it is because 
```
    // Eat the comma or space if there is one.
    trySkipToken(AsmToken::Comma);
```
in `AMDGPUAsmParser::parseInstruction`

This will also work (no commas)
`echo "exp mrt0 v0 v1 v0 v0 done" | ./bin/llvm-mc -triple=amdgcn -mcpu=gfx1200 -show-encoding`

https://github.com/llvm/llvm-project/pull/181641