[MVerifier] Don't check liveness of any debug instruction operands

Shiny new DBG_PHI instruction usually have physical registers as operands
-- however, the machine verifier checks to see whether they're live, and
occasionally this fails. There's a filter for DBG_VALUE instructions to not
get verified in this way: expand it to exempt all debug instructions from
liveness checking, which means DBG_PHIs get treated like DBG_VALUEs.

This also future proofs against us adding new debug instructions.

Differential Revision: https://reviews.llvm.org/D117891