RegisterCoalescer: Do not look for regclass of IMPLICIT_DEF.

IMPLICIT_DEF is a generic instruction and has no (fixed) output register
class defined. The rematerialization code of the register coalescer
should not scan the instruction description for a register class.

This fixes a problem showing up in
test/CodeGen/R600/subreg-coalescer-crash.ll with subregister liveness
enabled.

llvm-svn: 229443