X86-FMA3: Improved/enabled the memory folding optimization for scalar loads

generated for _mm_losd_s{s,d}() intrinsics and used in scalar FMAs generated 
for FMA intrinsics _mm_f{madd,msub,nmadd,nmsub}_s{s,d}().

Reviewer: David Kreitzer
Differential Revision: http://reviews.llvm.org/D14762

llvm-svn: 254140