[WebAssembly] Fix bugs in BLOCK/TRY placement

Summary:
Before we placed all TRY/END_TRY markers before placing BLOCK/END_BLOCK
markers. This couldn't handle this case:
```
bb0:
  br bb2
bb1:          // nearest common dominator of bb3 and bb4
  br_if ... bb3
  br bb4
bb2:
  ...
bb3:
  call @foo   // unwinds to ehpad
bb4:
  call @bar   // unwinds to ehpad
ehpad:
  catch
  ...
```

When we placed TRY markers, we placed it in bb1 because it is the
nearest common dominator of bb3 and bb4. But because bb0 jumps to bb2,
when we placed block markers, we ended up with interleaved scopes like
```
block
try
end_block
catch
end_try
```
which was not correct.

This patch fixes the bug by placing BLOCK and TRY markers in one pass
while iterating BBs in a function. This also adds some more routines to
`placeTryMarkers`, because we now have to assume that there can be
previously placed BLOCK and END_BLOCK.

Reviewers: dschuff

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59739

llvm-svn: 357007