[X86][Skylake] Merge multiple InstrRW entries that map to the same...

[X86][Skylake] Merge multiple InstrRW entries that map to the same SchedWriteRes group (NFCI) (PR35955)

I've also merged some VEX/non-VEX instregex strings with a (V?) prefix or (Y?) ymm variant - there are still a lot more of these to do.

This reduces the size of the optimized llc binary on my computer by 400K. Presumably because we went from 5000+ scheduler classes per CPU to ~2000.

llvm-svn: 328179