[clang] [llvm] [InstCombine] remove dead loads, such as memcpy from undef (PR #143958)

[Nikita Popov via llvm-commits]

nikic wrote:

https://github.com/dtcxzyw/llvm-opt-benchmark/pull/2433 has a few cases that regress for a reason that is not immediately obvious to me, if you want to take a look. (But this is overwhelmingly an improvement, so I don't particularly care.)

There are a number of test failures for asan and msan. I think we will want to disable this variant of the transform at least for sanitize_memory, because this will obviously hide uninitialized memory reads. Less sure about asan, but might be simplest to just disable it for it as well.

https://github.com/llvm/llvm-project/pull/143958