Introduce the TargetInstrInfo::KILL machine instruction and get rid of the

Introduce the TargetInstrInfo::KILL machine instruction and get rid of the
unused DECLARE instruction.

KILL is not yet used anywhere, it will replace TargetInstrInfo::IMPLICIT_DEF
in the places where IMPLICIT_DEF is just used to alter liveness of physical
registers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@83006 91177308-0d34-0410-b5e6-96231b3b80d8