MIPS: Optimize simulator.

MIPS: Optimize simulator.

The patch decreases the calls of huge switch instructions making the DecodeType*() functions to work in one phase and optimizing Instruction::InstructionType(). Speed gain in release full check is about 33% (6:13 s -> 4:09 s) and in optdebug full test is about 50% (12:29 -> 6:17)

BUG=

Review URL: https://codereview.chromium.org/1310883005

Cr-Commit-Position: refs/heads/master@{#30596}