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

[Jameson Nash via llvm-commits]

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vtjnash wrote:

Sorry for the continued stream of comments, but it turns out this test is supposed to crash, per 41895843b5915bb78e9d02aa711fa10f7174db43, so I've removed the XFAIL and fixed the test invocation to avoid the crash

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