Add some DAG combines for SUBC/SUBE. If nothing uses the carry/borrow out of...

Add some DAG combines for SUBC/SUBE. If nothing uses the carry/borrow out of subc, turn it into a sub. Turn (subc x, x) into 0 with no borrow. Turn (subc x, 0) into x with no borrow. Turn (subc -1, x) into (xor x, -1) with no borrow. Turn sube with no borrow in into subc.

llvm-svn: 147728