[mips] Implement l[wd]c3, and s[wd]c3.

Summary:
These instructions were added in MIPS-I, and MIPS-II but were removed in
MIPS-III. Interestingly, GAS continues to accept them when assembling for
MIPS-III.

For the moment, these instructions will follow GAS and accept them for
MIPS-III and newer but this will be tightened up when the invalid-*.s
tests are added.

Depends on D3647

Reviewers: vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D3648

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208311 91177308-0d34-0410-b5e6-96231b3b80d8