Incremental improvement to the handling of the x86 "Jump if rCX Zero"

Incremental improvement to the handling of the x86 "Jump if rCX Zero"
instruction.  Added the 64-bit version "jrcxz" so it is recognized and also
added the checks for incorrect uses of "jcxz" in 64-bit mode and "jrcxz" in
32-bit mode.  Still to do is to correctly handle the encoding of the
instruction adding the Address-size override prefix byte, 0x67, when the width
of the count register is not the same as the mode the machine is running in.
Which for example means the encoding of "jecxz" depends if you are assembling
as a 32-bit target or a 64-bit target.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@105661 91177308-0d34-0410-b5e6-96231b3b80d8