- Adds WASM opcodes I32x4SConvertI16x8Low, I32x4SConvertI16x8High,
  I32x4UConvertI16x8Low, I32x4UConvertI16x8High, which unpack half of
  an I16x8 register into a whole I32x4 register, with signed or unsigned
  extension. Having separate Low/High opcodes works around the difficulty
  of having multiple output registers, which would be necessary if we unpacked
  the entire I16x8 register.

- Adds WASM opcodes I16x8SConvertI8x16Low, I16x8SConvertI8x16High,
  I16x8UConvertI8x16Low, I16x8UConvertI8x16High, similarly to above.

- Adds WASM opcodes I16x8SConvertI32x4, I16x8UConvertI32x4,
  I8x16SConvert16x8, I8x16UConvertI16x8, which pack two source registers
  into a single destination register with signed or unsigned saturation. These
  could have been separated into half operations, but this is simpler to
  implement with SSE, AVX, and is acceptable on ARM. It also avoids adding
  operations that only modify half of their destination register.

- Implements these opcodes for ARM.

LOG=N
BUG=v8:6020

Review-Url: https://codereview.chromium.org/2800523002
Cr-Commit-Position: refs/heads/master@{#44541}