Before doing expensive global analysis, check to make sure the pointer is

Before doing expensive global analysis, check to make sure the pointer is
not invalidated on entry and on exit of the block.  This fixes some N^2
behavior in common cases, and speeds up gcc another 5% to 22.35s.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@19910 91177308-0d34-0410-b5e6-96231b3b80d8