Fix a bug in instcombine for fmul in fast math mode.

The instcombine recognized pattern looks like:
a = b * c
d = a +/- Cst
or
a = b * c
d = Cst +/- a

When creating the new operands for fadd or fsub instruction following the related fmul, the first operand was created with the second original operand (M0 was created with C1) and the second with the first (M1 with Opnd0).

The fix consists in creating the new operands with the appropriate original operand, i.e., M0 with Opnd0 and M1 with C1.

llvm-svn: 176300