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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

> Or, if we do need to preserve bitcode compat, how to best achieve it? Perhaps we convert them into inline-asm in the bitcode upgrader?

Really, the question is whether we plan to completely drop support for the x86_mmx type (including inline asm operands/results).  If we don't, then there's not much reason to touch the LLVM IR builtins; there isn't any actual "code" outside of TableGen, so there's basically zero maintenance work.  If we do drop it, then autoupgrade becomes very complicated and not really useful.

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