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[WebAssembly] Fix a bug in removing unnecessary branches

Authored by aheejin on May 25 2020, 1:34 AM.

Description

[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

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