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

[Reid Kleckner via cfe-commits]

rnk wrote:

Thanks! I was going to say, surely this can't be right, I was pretty confident that sret pointers consume registers for these conventions, but it looks like I was mistaken. I think this bug and code dates to 661f35b0c54c79805e6b17944b155c111bc39ec3 from 2014, which I think made the C++ and "C" sret return paths similar.

You can see from this example that when C++ rules force the use of an sret indirect return, a register is consumed:
https://godbolt.org/z/W3fdn8s5Y

I think this reveals how there is more separation between the C++ and C layers of the MSVC compiler than we have in Clang. We've seen similar examples of this kind of issue on AAarch64, where we have to choose between X0 and X8 depending on whether we're doing sret for C++ reasons or architectural reasons.

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