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}