X86: change zext moves to use sub-register infrastructure.

32-bit writes on amd64 zero out the high bits of the corresponding 64-bit
register. LLVM makes use of this for zero-extension, but until now relied on
custom MCLowering and other code to fixup instructions. Now we have proper
handling of sub-registers, this can be done by creating SUBREG_TO_REG
instructions at selection-time.

Should be no change in functionality.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182921 91177308-0d34-0410-b5e6-96231b3b80d8