Add explicit lowered type.
Distinguish operations for low/high.
Shl + Shr is equal to Mask.
Enable WASM_SIMD_COMPILED_AND_LOWERED_TEST for unpacking.

Change-Id: I0492318deb7c0ea0f4b30975ab107eedd446314a
Reviewed-on: https://chromium-review.googlesource.com/1079989Reviewed-by: Aseem Garg <aseemgarg@chromium.org>
Commit-Queue: Jing Bao <jing.bao@intel.com>
Cr-Commit-Position: refs/heads/master@{#53654}