Add support for I8x16Add, I8x16AddSaturateS, I8x16Sub, I8x16SubSaturateS,
I8x16Mul, I8x16MaxS, I8x16MinS, I8x16Eq, I8x16Ne, I8x16LtS,
I8x16LeS, I8x16ShrU, I8x16AddSaturateU, I8x16SubSaturateU, I8x16MaxU,
I8x16MinU, I8x16LtU, I8x16LeU, S128And, S128Or, S128Xor, S128Not for
mips32 and mips64 architectures.

BUG=

Review-Url: https://codereview.chromium.org/2798853003
Cr-Commit-Position: refs/heads/master@{#45512}