[RegAllocGreedy] IMPLICIT_DEF values shouldn't prefer registers

It costs nothing to spill an IMPLICIT_DEF value (the only spill code that's
generated is a KILL of the value), so when creating split constraints if the
live-out value is IMPLICIT_DEF the exit constraint should be DontCare instead
of PrefReg.

Differential Revision: https://reviews.llvm.org/D55652

llvm-svn: 349151