[DebugInfo] Correctly handle arrays with 0-width elements in GEP salvaging

Fixes an issue where GEP salvaging did not properly account for GEP
instructions which stepped over array elements of width 0 (effectively a
no-op). This unnecessarily produced long expressions by appending
`... + (x * 0)` and potentially extended the number of SSA values used
in the dbg.value. This also erroneously triggered an assert in the
salvage function that the element width would be strictly positive.
These issues are resolved by simply ignoring these useless operands.

Reviewed By: aprantl

Differential Revision: https://reviews.llvm.org/D111809