feat: Allows Simd constructors accept more than just constants

[MaxGraey]

Before we could only accept constants due to lowering to v128.const only. In this PR simd constructions now accept variables as well. All this lower to splat or const + replace_line-s as appropriate. For example:

// mixed const and vars arguments
const v1 = f32x4(0.0, 2.0, fx, fy); // where fx, fy non-precomputed expressions
// only var arguments
const v2 = i64x2(x, y);

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[dcodeIO]

Thanks :)