[clang] [llvm] Remove mmx 3dnow (PR #96246)

[Simon Pilgrim via cfe-commits]

RKSimon wrote:

> > So can we get all the mmx patches done in time for the 19.0 branch?
> 
> Maybe? Depends on how the discussion on the other PR goes -- whether that proposal is acceptable, or if a different transition strategy is desired.

Apart from #96540 how many more patches do you envision (still need to remove all the llvm side of things, plus the actual MMX builtins in BuiltinsX86.def - what else?). Should we just put all of that in #96540?

https://github.com/llvm/llvm-project/pull/96246