[clang] [X86][vectorcall] Do not consume register for indirect return value (PR #97939)

[Phoebe Wang via cfe-commits]

phoebewang wrote:

> It looks like MSVC also applies this rule to fastcall.

Good catch, done!
 
> Maybe put a boolean in the "state" to try to group together the code for specific conventions, instead of directly checking the CC.

There are 3 special conventions here: vectorcall, fastcall and regcall. We sometime check them all, sometime check one or two of them. I think it's impossible to use one boolean to group them.

https://github.com/llvm/llvm-project/pull/97939