For I16x8Splat and I8x16Splat, the arguments takes I32, which can hold a
value larger than what should be splatted. We add tests to check that
the splatted values is the truncated I32 value (top bits masked off).

See https://github.com/WebAssembly/simd/pull/151 for the updated to
proposal text.

Change-Id: Ib32770872e70c7cde2028130d2b44b416594610e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1986200Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65642}