Over eager combing prevents the correct folding of writemasks.
At the moment this occurs for ALL EVEX shuffles, in the future we need to check that the user of the root shuffle is a VSELECT that can fold to a writemask.
llvm-svn: 279934
An identity COPY like this:
%AL = COPY %AL, %EAX<imp-def>
has no semantic effect, but encodes liveness information: Further users
of %EAX only depend on this instruction even though it does not define
the full register.
Replace the COPY with a KILL instruction in those cases to maintain this
liveness information. (This reverts a small part of r238588 but this
time adds a comment explaining why a KILL instruction is useful).
llvm-svn: 274952
