[llvm-exegesis] `ExegesisX86Target::setRegTo()`: support mask (K) regs

This only supports the obvious case, where the requested width
is supported by the available ISA sed, and there is
an appropriate, or wider, `KMOV?` instruction.

This doesn't deal with the 32/64 bit mask without BW instruction set.

This was the missing functionality that was
causing crashes in fd52305f.