[X86] Avoid folding scalar loads into unary sse intrinsics

Not folding these cases tends to avoid partial register updates:
sqrtss (%eax), %xmm0
Has a partial update of %xmm0, while
movss (%eax), %xmm0
sqrtss %xmm0, %xmm0
Has a clobber of the high lanes immediately before the partial update,
avoiding a potential stall.

Given this, we only want to fold when optimizing for size.
This is consistent with the patterns we already have for some of
the fp/int converts, and in X86InstrInfo::foldMemoryOperandImpl()

Differential Revision: http://reviews.llvm.org/D15741

llvm-svn: 256671