[X86] Add pseudo instructions to use MULX with a single destination when the low result isn't used.

The instruction is defined to only produce high result if both
destinations are the same. We can exploit this to avoid
unnecessarily clobbering a register.

In order to hide this from register allocation we use a pseudo
instruction and expand the result during MCInst creation.

Differential Revision: https://reviews.llvm.org/D80500