- Increase FPUregisters_[] element size to 128b in order to support MSA regs
- Add skeleton for MSA instr. decoding in mips32 and mips64 simulator
- Add support for fill.df, copy_u.df and copy_s.df MSA instructions
- Assembler test for fill.df, copy_u.df and copy_s.df

BUG=

Review-Url: https://codereview.chromium.org/2799923002
Cr-Commit-Position: refs/heads/master@{#45529}