[x86] extend machine combiner reassociation optimization to SSE scalar adds

Extend the reassociation optimization of http://reviews.llvm.org/rL240361 (D10460)
to SSE scalar FP SP adds in addition to AVX scalar FP SP adds.

With the 'switch' in place, we can trivially add other opcodes and test cases in
future patches.

Differential Revision: http://reviews.llvm.org/D10975

llvm-svn: 241515