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

llvm-svn: 97673

llvm-svn: 97667

Machine CSE work in progress. It's doing some CSE now. But implicit def of physical registers are getting in the way.

llvm-svn: 97664

llvm-svn: 97635

llvm-svn: 97577

llvm-svn: 97543