[llvm] [MemCpyOpt] Merge memset and skip unrelated clobber in one scan (PR #90350)

[via llvm-commits]

XChy wrote:

@nikic, any advice on this? I guess there are mainly two overhead. One is the cost of `getUnderlyingObject` for non-candidates, which could be resolved by early-exit. One is the removal cost of MapVector, which is difficult to solve. If we use DenseMap directly, the order of flushed memset becomes indeterministic.

https://github.com/llvm/llvm-project/pull/90350