[WebAssembly] Fix bugs in rethrow depth counting and InstPrinter

Summary:
EH stack depth is incremented at `try` and decremented at `catch`. When
there are more than two catch instructions for a try instruction, we
shouldn't count non-first catches when calculating EH stack depths.

This patch fixes two bugs:
- CFGStackify: Exclude `catch_all` in the terminate catch pad when
  calculating EH pad stack, because when we have multiple catches for a
  try we should count only the first catch instruction when calculating
  EH pad stack.
- InstPrinter: The initial intention was also to exclude non-first
  catches, but it didn't account nested try-catches, so it failed on
  this case:
```
try
  try
  catch
  end
catch    <-- (1)
end
```
In the example, when we are at the catch (1), the last seen EH
instruction is not `try` but `end_try`, violating the wrong assumption.

We don't need these after we switch to the second proposal because there
is gonna be only one `catch` instruction. But anyway before then these
bugfixes are necessary for keep trunk in working state.

Reviewers: dschuff

Subscribers: sbc100, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D53819

llvm-svn: 346029