canonicalize a v2f64 blendi of 2 registers

This canonicalization step saves us 3 pattern matching possibilities * 4 math ops
for scalar FP math that uses xmm regs. The backend can re-commute the operands
post-instruction-selection if that makes register allocation better.

The tests in llvm/test/CodeGen/X86/sse-scalar-fp-arith.ll cover this scenario already,
so there are no new tests with this patch.

Differential Revision: http://reviews.llvm.org/D7777

llvm-svn: 230024