[X86] Special-case 2x CMOV when custom-inserting.

This lets us avoid a few copies that are otherwise hard to get rid of.
The way this is done is, the custom-inserter looks at the following
instruction for another CMOV, and replaces both at the same time.
A previous version used a new CMOV2 opcode, but the custom inserter
is expected to be able to return a different basic block anyway, which
means it's OK - though far from ideal - to alter that block's contents.
Explicitly document that, in case it ever makes a difference.
Alternatives welcome!

Follow-up to r231045.

rdar://19767934
Closes http://reviews.llvm.org/D8019

llvm-svn: 231046