Re-apply r97667 but with a little bit of thought put into the patch. This...

Re-apply r97667 but with a little bit of thought put into the patch. This implements a special DenseMapInfo trait for DenseMap<MachineInstr*> that compare the value of the MachineInstr rather than the pointer value. Since the hashing and equality test functions ignore defs it's useful for doing CSE kind optimization.

llvm-svn: 97678