[PowerPC] better instruction selection for OR (XOR) with a 32-bit immediate

On PPC64, OR (XOR) with a 32-bit immediate can be done with only two instructions, i.e. ori + oris.
But the current LLVM generates three or four instructions for this purpose (and also it clobbers one GPR).

This patch makes PPC backend generate ori + oris (xori + xoris) for OR (XOR) with a 32-bit immediate.

e.g. (x | 0xFFFFFFFF) should be

	ori 3, 3, 65535
	oris 3, 3, 65535

but LLVM generates without this patch

	li 4, 0
	oris 4, 4, 65535
	ori 4, 4, 65535
	or 3, 3, 4

Differential Revision: https://reviews.llvm.org/D34757

llvm-svn: 311526