X86 MC: Handle instructions like fxsave that match multiple operand sizes

Instructions like 'fxsave' and control flow instructions like 'jne'
match any operand size. The loop I added to the Intel syntax matcher
assumed that using a different size would give a different instruction.
Now it handles the case where we get the same instruction for different
memory operand sizes.

This also allows us to remove the hack we had for unsized absolute
memory operands, because we can successfully match things like 'jnz'
without reporting ambiguity.  Removing this hack uncovered test case
involving 'fadd' that was ambiguous. The memory operand could have been
single or double precision.

llvm-svn: 216604