[wasm] Refactor {unreachable} validation

In the existing code, whenever unreachable control instructions needed
values from the stack which were not available, values of type kWasmVar
were put on the stack. When these values were type-checked the first
time, the expected type was assigned to them for later validation. This
behavior has several draw-backs:
* In an unobservable way, this implementation does not match the
  requirements of the spec. With the anyref proposal, this difference
  becomes observable.
* Type checking functions were not read-only anymore, because if
  unreachable code was validated, the stack got manipulated in these
  functions.

With the refactoring, I pulled out the handling of unreachable code
out of the type checking functions. These checking functions can be
validation-only functions.

For type checking unreachable code, I start by popping values of the
expected types off the stack. Thereby all available values on the stack
get type-checked. Afterwards, I push all values again on the stack with
the expected type if needed. This allows to continue the expected type
checking for later instructions.

R=clemensh@chromium.org

Bug: v8:7581
Change-Id: Ib98e70a44bf9780626d4aa8a3e5fe8c2f230b787
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1645328Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62203}