[mips][micromips] Make getPointerRegClass() result depend on the instruction.

Summary:
Previously, it returned the GPR16MMRegClass for all instructions which was
incorrect for instructions like lwsp/lwgp and unnecesarily restricted the
permitted registers for instructions like lw32.

This fixes quite a few of the -verify-machineinstrs errors reported in PR27458.
I've only added -verify-machineinstrs to one test in this change since I
understand there is a plan to enable the verifier by default.

Reviewers: hvarga, zbuljan, zoran.jovanovic, sdardis

Subscribers: dsanders, llvm-commits, sdardis

Differential Revision: http://reviews.llvm.org/D19873

llvm-svn: 268918