Fix a latent bug exposed by my truncstore patch. We compiled stfiwx-2.ll to:

_test:
	fctiwz f0, f1
	stfiwx f0, 0, r4
	blr 

instead of:

_test:
	fctiwz f0, f1
	stfd f0, -8(r1)
	nop
	nop
	lwz r2, -4(r1)
	stb r2, 0(r4)
	blr 

The former is not correct (stores 4 bytes, not 1).

llvm-svn: 46161