[wasm-simd][scalar-lowering] Use switch for SimdType conversions

Use a doubly-nested switch on SimdType for conversions, this ensures
that we handle all possible cases (and we actually missed one,
converting i64x2 -> f32x4, which is added in this patch.)

Bug: v8:10507
Change-Id: I493becb2616c51d02d5868f235653baba5a0b4af
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2464144
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70453}