MIPS: Major fixes and clean-up in asm. for instruction encoding.

MIPS: Major fixes and clean-up in asm. for instruction encoding.

- Fixed single float register type instruction en[de]coding in assembler and disassembler.
- Added max and min instructions for r6 and corresponding tests.
- Fixed selection instruction for boundary cases in simulator.
- Update assembler tests to be more thorough wrt boundary cases.

TEST=cctest/test-assembler-mips64/MIPS17, MIPS18
     cctest/test-disasm-mips64/Type1
     cctest/test-assembler-mips/MIPS16, MIPS17
     cctest/test-disasm-mips/Type1
BUG=

Review URL: https://codereview.chromium.org/1057323002

Cr-Commit-Position: refs/heads/master@{#27601}