Lower unsigned vsetcc to psubus in certain cases

The current approach to lower a vsetult is to flip the sign bit of the
operands, swap the operands and then use a (signed) pcmpgt.  psubus (unsigned
saturating subtract) can be used to emulate a vsetult more efficiently:

+    case ISD::SETULT: {
+      // If the comparison is against a constant we can turn this into a
+      // setule.  With psubus, setule does not require a swap.  This is
+      // beneficial because the constant in the register is no longer
+      // destructed as the destination so it can be hoisted out of a loop.

I also enable lowering via psubus in a few other cases where it's clearly
beneficial: setule and setuge if minu/maxu cannot be used.

rdar://problem/14338765

Patch by Adam Nemet <anemet@apple.com>.