RegisterCoalescer: Fix implicit def handling in register coalescer

If liveranges induced by an IMPLICIT_DEF get completely covered by a
proper liverange the IMPLICIT_DEF instructions and its corresponding
definitions have to be removed from the live ranges. This has to happen
in the subregister live ranges as well (I didn't see this case earlier
because in most programs only some subregisters are covered and the
IMPLCIT_DEF won't get removed).

No testcase, I spent hours trying to create one for one of the public
targets, but ultimately failed because I couldn't manage to properly
control the placement of COPY and IMPLICIT_DEF instructions from an .ll
file.

llvm-svn: 233217