[InstCombine][DebugInfo] Fold constants wrapped in metadata

Summary:
When constant folding, constants that are wrapped in metadata were not
folded. This could lead to dbg.values being the only user of a constant
expression, due to the non-dbg uses having been rewritten, resulting in
the constant later on being removed by some other pass. This occurred
with the attached test case, in which the non-rewritten GEP in the
dbg.value intrinsic was later on removed by globalopt.

This patch makes the code look through metadata and fold such constants.

I guess that we in the future may want to allow dbg.values using GEPs and
other constant expressions to be emittable even if there are no non-dbg
uses, but for example SelectionDAG does not support that.

Reviewers: jmorse, aprantl, vsk, davide

Reviewed By: aprantl, vsk, davide

Subscribers: hiraditya, llvm-commits

Tags: #debug-info, #llvm

Differential Revision: https://reviews.llvm.org/D73630