[X86] Emit 11-byte or 15-byte NOPs on recent AMD targets, else default to 10-byte NOPs (PR22965)

We currently emit up to 15-byte NOPs on all targets (apart from Silvermont), which stalls performance on some targets with decoders that struggle with 2 or 3 more '66' prefixes.

This patch flags recent AMD targets (btver1/znver1) to still emit 15-byte NOPs and bdver* targets to emit 11-byte NOPs. All other targets now emit 10-byte NOPs apart from SilverMont CPUs which still emit 7-byte NOPS.

Differential Revision: https://reviews.llvm.org/D42616

llvm-svn: 323693