MC/X86: Subdivide immediates a bit more, so that we properly recognize...

MC/X86: Subdivide immediates a bit more, so that we properly recognize immediates based on the width of the target instruction. For example:
  addw $0xFFFF, %ax
should match the same as
  addw $-1, %ax
but we used to match it to the longer encoding.

llvm-svn: 104453