[mips] Preparatory work for a generic scheduler

[mips] Preparatory work for a generic scheduler

Extend instruction definitions from nearly all ISAs to include
appropriate instruction itineraries. Change MIPS16s gp prologue
generation to use real instructions instead of using a pseudo
instruction.

Reviewers: dsanders, vkalintiris

Differential Review: https://reviews.llvm.org/D23548

llvm-svn: 279623