[AsmWriter] Factor out mnemonic generation to accessible getMnemonic.

This patch factors out the part of printInstruction that gets the
mnemonic string for a given MCInst. This is intended to be used
subsequently for the instruction-mix remarks to display the final
mnemonic (D90040).

Unfortunately making `getMnemonic` available to the AsmPrinter
seems to require making it virtual. Not sure if there's a way around
that with the current layering of the AsmPrinters.

Reviewed By: Paul-C-Anagnostopoulos

Differential Revision: https://reviews.llvm.org/D90039