[WebAssembly] Fix a bug in removing unnecessary branches

Summary:
One of the things `removeUnnecessaryInstrs()` in CFGStackify does is to
remove an unnecessary unconditinal branch before an EH pad. When there
is an unconditional branch right before a catch instruction and it
branches to the end of `end_try` marker, we don't need the branch,
because it there is no exception, the control flow transfers to
that point anyway.
```
bb0:
  try
    ...
    br bb2      <- Not necessary
bb1:
  catch
    ...
bb2:
  end
```

This applies when we have a conditional branch followed by an
unconditional one, in which case we should only remove the unconditional
branch. For example:
```
bb0:
  try
    ...
    br_if someplace_else
    br bb2                 <- Not necessary
bb1:
  catch
    ...
bb2:
  end
```

But `TargetInstrInfo::removeBranch` we used removed all existing
branches when there are multiple ones. This patch fixes it by only
deleting the last (= unconditional) branch manually.

Also fixes some `preds` comments in the test file.

Reviewers: dschuff

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80572