Instead of callign removeTriviallyDeadNodes on the global graph every time

Instead of callign removeTriviallyDeadNodes on the global graph every time
removeDeadNodes is called, only call it at the end of the pass being run.
This saves 1.3 seconds running DSA on 177.mesa (5.3->4.0s), which is
pretty big.  This is only possible because of the automatic garbage
collection done on forwarding nodes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@11178 91177308-0d34-0410-b5e6-96231b3b80d8