[PowerPC] Zero-extend the compare operand for ATOMIC_CMP_SWAP

[PowerPC] Zero-extend the compare operand for ATOMIC_CMP_SWAP

Part of the fix for https://bugs.llvm.org/show_bug.cgi?id=35812.
This patch ensures that the compare operand for the atomic compare and swap
is properly zero-extended to 32 bits if applicable.
A follow-up commit will fix the extension for the SETCC node generated when
expanding an ATOMIC_CMP_SWAP_WITH_SUCCESS. That will complete the bug fix.

Differential Revision: https://reviews.llvm.org/D41856

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@322372 91177308-0d34-0410-b5e6-96231b3b80d8