[PATCH] [RewriteStatepointsForGC] Preprocess the IR to remove unreachable blocks and single entry phis

[Philip Reames]

Hi sanjoy, swaroop.sridhar,

Two related small changes:
- Various dominance based queries about liveness can get confused if we're talking about unreachable blocks.  To avoid reasoning about such cases, just remove them before rewriting statepoints.
- Remove single entry phis (likely left behind by LCSSA) to reduce the number of live values.

Both of these are motivated by http://reviews.llvm.org/D8674, but are not actually dependent on that.

http://reviews.llvm.org/D8675

Files:
  lib/Transforms/Scalar/RewriteStatepointsForGC.cpp
  test/Transforms/RewriteStatepointsForGC/preprocess.ll

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