[PATCH] D136321: [Assignment Tracking Analysis][2/*] Remove redundant location definitions

[Orlando Cazalet-Hyams via Phabricator via llvm-commits]

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Implement the `removeRedundantDbgInstrs` cleanup from `lib/Transforms/Utils/BasicBlockUtils.cpp` for `FunctionVarLocBuilder`'s variable location definitions.

Removing redundant entries has the obvious benefits of reducing peak memory when using the results of the analysis and reducing the final number of MIR debug instructions.

It also has a less obvious benefit. This patch is not an NFC change because SelectionDAG has a habit of doing Bad Things with variable locations - removing the number of location definitions tends to reduce the number of Bad Things.

Two examples I have seen:

1. Argument `SDDbgValue`s hoisted above undef ``SDDbgValue`s.
2. undef `SDDbgValue`s in a block of contiguous `SDDbgValue`s spuriously changing order.


https://reviews.llvm.org/D136321

Files:
  llvm/lib/CodeGen/AssignmentTrackingAnalysis.cpp

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