Disable some unsafe-fp-math DAG-combine transformation after legalization.

For instance, following transformation will be disabled:
    x + x + x => 3.0f * x;

The problem of these transformations is that it introduces a FP constant, which
following Instruction-Selection pass cannot handle.

Reviewed by Nadav, thanks a lot!

rdar://13445387

llvm-svn: 177933