MIPS: Add float instructions and test coverage, part one

MIPS: Add float instructions and test coverage, part one

Implement assembler, disassembler tests for all instructions for mips32 and mips64. Additionally, add missing single precision float instructions for r2 and r6 architecture variants in assembler, simulator and disassembler with corresponding tests.

Review URL: https://codereview.chromium.org/1119203003

Cr-Commit-Position: refs/heads/master@{#28402}