RegisterCoalescer: Fix verifier error on redef of subregister for live out implicit_defs

A live out implicit_def wasn't deleted, but the subranges weren't
correctly updated. The main range was correct but the def
corresponding to the initial main range def instruction was missing
from the lanes redefined in another block.

The written lanes are not quite the same as the valid lanes in the
case of an implicit_def.

Fixes verifier error in blender. There is an additional verifier in
some of the testcase variants where an empty subrange remains.