[llvm] [NFC][SPIRV] Fix function type recovery (PR #165934)

[Alex Voicu via llvm-commits]

https://github.com/AlexVlx commented:

> Is this TODO still up to date now? 

I believe it is not, however I was / am not certain if the intent is to eventually allow multiple mutations per argument (unclear how that would look but presumably one could nest within a MD operand?), rather than single mutations for multiple arguments. However, looking at it, it might simply be confusing at this point, so I think I'll remove it and we can revisit.

https://github.com/llvm/llvm-project/pull/165934