[RemoveDIs] Fix removeRedundantDdbgInstrs utils for dbg.declares (#74102)

The intrinsic variants of these functions don't do anything to
dbg.declares so the non-instruction variants should ignore them too.

Tested in llvm/test/DebugInfo/duplicate_dbgvalue.ll, which has
`--try-experimental-debuginfo-iterators` added in #73504.

The tests will become "live" once #74090 lands (see for more info).