[mips] Update the P5600 scheduler model not to use instruction itineraries.

This mostly brings the P5600 scheduler model to a mostly complete
status. There are a number of instructions which trigger the
`error:'MipsP5600Model' lacks information for` error. These are certain
codegen only instructions relating to MIPS64 which can be addressed by
using the correct predicates for them. That will be done in a full-up
patch.

Patch by Simon Dardis.

Differential revision: https://reviews.llvm.org/D45245

llvm-svn: 336802