[X86] Remove unnecessary COPY from EmitLoweredCascadedSelect.

I believe we already checked that the destination of the first
CMOV is only used by the second CMOV so I don't think there is any
reason we need the PHI to write the register that was used by the
first CMOV. We can directly use the second CMOV destination and
avoid the copy.

This may be a left over from when the cascaded select handling
was part of the main algorithm before it was refactored in D35685.

Reviewed By: pengfei

Differential Revision: https://reviews.llvm.org/D128124