[WebAssembly] Use v128.const instead of splats for constants

We previously used splats instead of v128.const to materialize vector constants
because V8 did not support v128.const. Now that V8 supports v128.const, we can
use v128.const instead. Although this increases code size, it should also
increase performance (or at least require fewer engine-side optimizations), so
it is an appropriate change to make.

Differential Revision: https://reviews.llvm.org/D100716