[X86] When pattern-matching scalar FMA3 intrinsics, don't re-arrange the first and second operands.

The semantics of the scalar FMA intrinsics are that the high vector elements are copied from the first source.
The existing pattern switches src1 and src2 around, to match the "213" order, which ends up tying the original src2 to the dest. Since the actual scalar fma3 instructions copy the high elements from the dest register, the wrong values are copied.

This modifies the pattern to leave src1 and src2 in their original order.

Differential Revision: http://reviews.llvm.org/D9908

llvm-svn: 238131