Implements lowering for:
- i16x8.load8x8_s
- i16x8.load8x8_u
- i32x4.load16x4_s
- i32x4.load16x4_u

As before, i64x2 is not implemented since 64-bit lowering and scalar
lowering don't work together yet.

Bug: v8:9886
Change-Id: I3728d009e053acf82baacbcf1c6c08ea636ef241
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2044546Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66380}