Remove extra truncs/exts around i32 bit operations on PPC64

This generalizes the code to eliminate extra truncs/exts around i1 bit
operations to also do the same on PPC64 for i32 bit operations. This eliminates
a fairly prevalent code wart:

int foo(int a) {
  return a == 5 ? 7 : 8;
}

On PPC64, because of the extension implied by the ABI, this would generate:

	cmplwi 0, 3, 5
	li 12, 8
	li 4, 7
	isel 3, 4, 12, 2
	rldicl 3, 3, 0, 32
	blr

where the 'rldicl 3, 3, 0, 32', the extension, is completely unnecessary. At
least for the single-BB case (which is all that the DAG combine mechanism can
handle), this unnecessary extension is no longer generated.