Fix a significant code quality regression I introduced on PPC64 quite

a while ago.  We now produce:

_foo:
	mflr r0
	std r0, 16(r1)
	ld r2, 16(r1)
	std r2, 0(r3)
	ld r0, 16(r1)
	mtlr r0
	blr 

instead of:

_foo:
	mflr r0
	std r0, 16(r1)
	lis r0, 0
	ori r0, r0, 16
	ldx r2, r1, r0
	std r2, 0(r3)
	ld r0, 16(r1)
	mtlr r0
	blr 

for:

void foo(void **X) {
  *X = __builtin_return_address(0);
}

on ppc64.

llvm-svn: 44701