Compile time decreasing in the case we're dealing with Machine Combiner.

Before this patch compile time was about 21s (see below). After this patch
we have less than 2s (see bellow).

  Intel(R) Xeon(R) CPU E5-2676 v3 @ 2.40GHz

    DAGCombiner - trunk
        time ./llc spill_fdiv.ll -o /dev/null -enable-unsafe-fp-math
        real  0m1.685s

    DAGCombiner + Speed patch
        time ./llc spill_fdiv.ll -o /dev/null -enable-unsafe-fp-math
        real  0m1.655s

    MachineCombiner w/o Speed patch
        time ./llc spill_fdiv.ll -o /dev/null -enable-unsafe-fp-math
        real  0m21.614s

    MachineCombiner + Speed patch
        time ./llc spill_fdiv.ll -o /dev/null -enable-unsafe-fp-math
        real  0m1.593s

The test spill_fdiv.ll  is attached to D29627
D29627 should be closed.

llvm-svn: 294936