A single AArch64 SIMD signed/unsigned Shift Right and Accumulate can be
used to implement Wasm SIMD add(shr(x, imm), y). This gives a 1-1.5%
improvement on some compute intensive Wasm benchmarks on Neoverse-N1.

Mla and Adalp optimisations were refactored to match the style of the
added code.

Change-Id: Id5959a31ca267e02b7d60e7ff6f942adb029b41e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3089157Reviewed-by: Zhi An Ng <zhin@chromium.org>
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Cr-Commit-Position: refs/heads/master@{#76280}