[llvm] [MemCpyOpt] Continue merging memset with unrelated clobber (PR #89550)

[via llvm-commits]

XChy wrote:

> New compile-time numbers: https://llvm-compile-time-tracker.com/compare.php?from=ba702aa067c0659d2263d2144d38666efa4a1750&to=965d72d4a9918e00a02609c40f31d00d8fe63d64&stat=instructions:u

The stage1 impact looks obvious. If you are not happy with the compile-time, I will try to avoid revisiting the ones to the same object in this patch.
I have a basic idea in mind to process one basic block in one pass now: 
- maintain a map, whose key is the underlying object, and the value is corresponding MemsetRanges
- traverse the instructions one by one
- once a clobber appears
   - for a store/memset, if the map contains the corresponding underlying object, try to insert it into the MemsetRanges. Otherwise, we flush all may-aliased MemsetRanges, and insert a new MemsetRanges into the map.
   - for other clobbers, we flush the aliased ranges in the map, this requires checking every MemsetRanges in the map.

This method avoids revisiting the ones to the same object and the ones don't read or write memory.
But anyway, for a long non-aliased store sequence, it's still quadratic.



https://github.com/llvm/llvm-project/pull/89550