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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

I'd prefer we rip off ALL the 3dnow/mmx bandaid in one big series of patches - and not split across the 19.X/20.X releases. 

So can we get all the mmx patches done in time for the 19.0 branch?

Also, this "might" have bigger impact than the KNL changes - should we announce the plan in 19.X and do it for 20.X?

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