[AsmPrinter] Use getMnemonic for instruction-mix remark.

This patch uses the new `getMnemonic` helper from D90039
to display mnemonics instead of the internal opcodes.

The main motivation behind using the mnemonics is that they
are more user-friendly and more directly related to the assembly
the users will be presented.

Reviewed By: paquette

Differential Revision: https://reviews.llvm.org/D90040