[DebugInfo] Remove redundant DebugLocEntry::MergeValues() function, NFC

[DebugInfo] Remove redundant DebugLocEntry::MergeValues() function, NFC

Summary:
The MergeValues() function would try to merge two entries if they shared
the same beginning label. Having the same beginning label means that the
former entry's range would be empty; however, after D55919 we no longer
create entries for empty ranges, so we can no longer land in a situation
where that check in MergeValues would succeed. Instead, the "merging" is
done by keeping the live values from the preceding empty ranges in
OpenRanges, and adding them to the first non-empty range.

Reviewers: aprantl, dblaikie, loladiro

Reviewed By: aprantl

Subscribers: llvm-commits

Tags: #debug-info, #llvm

Differential Revision: https://reviews.llvm.org/D59301

llvm-svn: 357974