[mips] Enable code generation for MIPS-III.

Summary:
This commit enables the MIPS-III target and adds support for code
generation of SELECT nodes. We have to use pseudo-instructions with
custom inserters for these nodes as MIPS-III CPUs do not have
conditional-move instructions.

Depends on D6212

Reviewers: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6464