[X86] In X86FastEmitSSESelect, fall back to SelectionDAG if the inputs to the...

[X86] In X86FastEmitSSESelect, fall back to SelectionDAG if the inputs to the compare can't be found in registers.

We were checking that the original Value * for the compare operands
were null. But that can never happen.

I believe we intended to check for 0 registers here instead.

Fixes PR44749.