[OPT-DBG] Teach DbgEntityHistoryCalculator about meta-instructions.
The calculator was considering instructions such as KILLs as clobbers
of a physical address. This is wrong as meta instructions such as KILLs
produce no output in the final program and thus don't clobber or change
any physical location's value. As a result they're safe to ignore whilst
calculating location list ranges.
reviewers: aprantl, vsk
diff revision: https://reviews.llvm.org/D70497