Local type information was stored in the {WasmFullDecoder}, and a
pointer to that vector was handed to {WasmDecoder}. Since
{WasmFullDecoder} inherits from {WasmDecoder}, we can just move the
vector to the {WasmDecoder} class, and save an indirection and an
unnecessary nullptr check.

Drive-by: Rename {GetLocalType} to {local_type}, since it's a simple
accessor.
Drive-by 2: Move fields of {WasmDecoder} to the end of the class, as
mandated in the style guide.
Drive-by 3: Rename some locals in the 'let' decoding to make the meaning
more clear.

R=thibaudm@chromium.org

Bug: v8:10576
Change-Id: I6ab9831f0c1955e47562e84c5fbf15807439b024
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2264360Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68541}