[x86] Add a reassociation optimization to increase ILP via the MachineCombiner pass

This is a reimplementation of D9780 at the machine instruction level rather than the DAG.

Use the MachineCombiner pass to reassociate scalar single-precision AVX additions (just a
starting point; see the TODO comments) to increase ILP when it's safe to do so.

The code is closely based on the existing MachineCombiner optimization that is implemented
for AArch64.

This patch should not cause the kind of spilling tragedy that led to the reversion of r236031.

Differential Revision: http://reviews.llvm.org/D10321

llvm-svn: 239486