[WASM] Use rounding_mul_shift_right for q15mulr_sat_s pattern

[rootjalex]

Noticed that the q15mulr_sat_s rule didn't use the corresponding intrinsic - meaning we'd lower rounding_mul_shift_right just to match the rule before.

[steven-johnson]

Do we need an updated test in simd_op_check?

[rootjalex]

No, this change should have not introduce any code generation change - it just makes code generation (slightly) faster.

find_intrinsics will have already lifted the target pattern to a rounding_mul_shift_right. Before, when CodeGen_LLVM lowers that intrinsic (because CodeGen_WebAssembly doesn't pattern match it), the exact expected pattern is reproduced and then matched to the pattern that has the lowered form. Now we just side-step the lowering.

[rootjalex]

Test failure appears unrelated