This change adds simd lowering support for:
I8x16Splat,I8x16ExtractLane,I8x16ReplaceLane,I8x16Neg,I8x16Shl,
I8x16ShrS,I8x16Add,I8x16AddSaturateS,I8x16Sub,I8x16SubSaturateS,
I8x16Mul,I8x16MinS,I8x16MaxS,I8x16ShrU,I8x16AddSaturateU,
I8x16SubSaturateU,I8x16MinU,I8x16MaxU,I8x16Eq,I8x16Ne,I8x16LtS,
I8x16LeS,I8x16LtU,I8x16LeU operations

BUG=

Review-Url: https://codereview.chromium.org/2867343002
Cr-Commit-Position: refs/heads/master@{#45331}