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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@104453 91177308-0d34-0410-b5e6-96231b3b80d8