Remove dead instructions before Redoing

Before reevaluating instructions, iterate over all instructions
to be reevaluated and remove trivially dead instructions and if
any of it's operands become trivially dead, mark it for deletion
until all trivially dead instructions have been removed

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256773 91177308-0d34-0410-b5e6-96231b3b80d8