[clang] [llvm] Clang: convert `__m64` intrinsics to unconditionally use SSE2 instead of MMX. (PR #96540)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

I guess the clang calling convention code never uses MMX types for passing/returning values?

Have you looked at the code quality?  #41665 mentions potential issues with widening vectors.

This doesn't touch inline asm or _mm_empty; I guess you're leaving that for a followup?

https://github.com/llvm/llvm-project/pull/96540