[x64] Separate out three-byte opcode decoding

Decoding of three-byte opcodes were within the two-byte decoding
function, separate it out, and fix an incorrect comment about us
no having any three-byte opcodes (that is no longer true).

Also un-nest a large if/else out into parent scope.

Test: out/x64.debug/cctest test-disasm-x64/DisasmX64 --random-seed=1

Bug: v8:10933
Change-Id: I494d67ac75cc4500d5f0045f1087b856e6375f82
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2477426
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70584}