This commit is a step toward enabling test-run-wasm-simd tests for MIPS.
36 of those were failing in V8 builtin simulator because some instructions
were not implemented.  Also there are minor fixes to some of the already
implemented instructions.

This commit has only 32-bit implementation. After review I will add
64-bit version.

Bug: 
Change-Id: I25b0cac352db3efb56b922ace64ab2aaef82472d
Reviewed-on: https://chromium-review.googlesource.com/744008Reviewed-by: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Cr-Commit-Position: refs/heads/master@{#49439}