DetectDeadLanes: Increase precision when detecting undef inputs

In case of COPY-like instruction we may be able to deduce that a certain
input is unused, based on the used lanes of the register defined by the
instruction.
This even works accross otherwise incompatible copies (no need to have
compatible lanemasks, completely unused operands are still completely
unused). It even makes sense to redo the analysis in this case since we
gained information for a case we previously stopped at because of the
incompatible masks.

llvm-svn: 268815