Make the "pruning cloner" smarter. As it propagates constants through the

Make the "pruning cloner" smarter.  As it propagates constants through the
code (while cloning) it often gets the branch/switch instructions.  Since it
knows that edges of the CFG are dead, it need not clone (or even look) at
the obviously dead blocks.  This should speed up the inliner substantially on
code where there are lots of inlinable calls to functions with constant
arguments.  On C++ code in particular, this kicks in.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@28641 91177308-0d34-0410-b5e6-96231b3b80d8