AMDGPU: Eliminate half of i64 or if one operand is zero_extend from i32

This helps clean up some of the mess when expanding unaligned 64-bit
loads when changed to be promote to v2i32, and fixes situations
where or x, 0 was emitted after splitting 64-bit ors during moveToVALU.

I think this could be a generic combine but I'm not sure.

llvm-svn: 266104