X86: use sub-register sequences for MOV*r0 operations
Instead of having a bunch of separate MOV8r0, MOV16r0, ... pseudo-instructions, it's better to use a single MOV32r0 (which will expand to "xorl %reg, %reg") and obtain other sizes with EXTRACT_SUBREG and SUBREG_TO_REG. The encoding is smaller and partial register updates can sometimes be avoided. Until recently, this sequence was a barrier to rematerialization though. That should now be fixed so it's an appropriate time to make the change. llvm-svn: 182928
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