Add support for I16x8Mul, I16x8MaxS, I16x8MinS, I16x8Eq, I16x8Ne,
I16x8LtS, I16x8LeS, I16x8AddSaturateU, I16x8SubSaturateU, I16x8MaxU,
I16x8MinU, I16x8LtU, I16x8LeU, I8x16Splat, I8x16ExtractLane,
I8x16ReplaceLane, I8x16Neg, I8x16Shl, I8x16ShrS, S16x8Select,
S8x16Select for mips32 and mips64 architectures.

BUG=

Review-Url: https://codereview.chromium.org/2791213003
Cr-Commit-Position: refs/heads/master@{#45312}