[WebAssembly] Handle block and polymorphic stack in AsmTypeCheck (#110770)

This makes the type checker handle blocks with input parameters and
return types, branches, and polymorphic stacks correctly.

We maintain the stack of "block info", which contains its input
parameter type, return type, and whether it is a loop or not. And this
is used when checking the validity of the value stack at the `end`
marker and all branches targeting the block.

`StackType` now supports a new variant `Polymorphic`, which indicates
the stack is in the polymorphic state. `Polymorphic`s are not popped
even when `popType` is executed; they are only popped when the current
block ends.

When popping from the value stack, we ensure we don't pop more than we
are allowed to at the given block level and print appropriate error
messages instead. Also after a block ends, the value stack is guaranteed
to have the right types based on the block return type. For example,
```wast
block i32
  unreachable
end_block
;; You can expect to have an i32 on the stack here
```

This also adds handling for `br_if`. Previously only `br`s were checked.

`checkEnd` and `checkBr` were removed and their contents have been
inlined to the main `typeCheck` function, because they are called only
from a single callsite.

This also fixes two existing bugs in AsmParser, which were required to
make the tests passing. I added Github comments about them inline.

This modifies several existing invalid tests, those that passed
(incorrectly) before but do not pass with the new type checker anymore.

Fixes #107524.