[mips] Support SELECT nodes for targets that don't have conditional-move instructions.

Summary:
For Mips targets that do not have conditional-move instructions, ie. targets
before MIPS32 and MIPS-IV, we have to insert a diamond control-flow
pattern in order to support SELECT nodes. In order to do that, we add
pseudo-instructions with a custom inserter that emits the necessary
control-flow that selects the correct value.

With this patch we add complete support for code generation of Mips-II targets
based on the LLVM test-suite.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6212