MC: Split the x86 asm matcher implementations by dialect

The existing matcher has lots of AT&T assembly dialect assumptions baked
into it.  In particular, the hack for resolving the size of a memory
operand by appending the four most common suffixes doesn't work at all.
The Intel assembly dialect mnemonic table has ambiguous entries, so we
need to try matching multiple times with different operand sizes, since
that's the only way to choose different instruction variants.

This makes us more compatible with gas's implementation of Intel
assembly syntax.  MSVC assumes you want byte-sized operations for the
instructions that we reject as ambiguous.

Reviewed By: grosbach

Differential Revision: http://reviews.llvm.org/D4747