Fix PR1845 and rdar://5676945. Generic vectors smaller

than hardware supported type will be scalarized, so we
can infer their alignment from that info.

We now codegen pr1845 into:

_boolVectorSelect:
	lbz r2, 0(r3)
	stb r2, -16(r1)
	blr 

llvm-svn: 45796