[SimplifyCFG] Don't kill empty cleanuppads with multiple uses

[SimplifyCFG] Don't kill empty cleanuppads with multiple uses

A basic block could contain:
  %cp = cleanuppad []
  cleanupret from %cp unwind to caller

This basic block is empty and is thus a candidate for removal.  However,
there can be other uses of %cp outside of this basic block.  This is
only possible in unreachable blocks.

Make our transform more correct by checking that the pad has a single
user before removing the BB.

This fixes PR28005.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@271816 91177308-0d34-0410-b5e6-96231b3b80d8