With the value stack refactoring in 1b5c7e15 / r73193, the
combination of helper functions called by PeekArgs() ended
up checking the stack height repeatedly. This CL avoids that
by introducing a ValidateArgType() helper that does not check
stack height.
Bonus: achieve a small speedup by special-casing two of the
most common opcodes in the decoder's main dispatcher.

Fixed: chromium:1185082
Change-Id: I6d51aca844ef9377d203147f74ff8137e12a23e7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2745341
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73294}