[llvm] [AggressiveInstCombine] Guard memset with length in [0, 1] (PR #213240)

[via llvm-commits]

hsnaveen2u wrote:

"Build and Test" failures reproduce identically on the unmodified main branch.
The only test-suite failure is llvm/test/Transforms/AggressiveInstCombine/trunc_vector_instrs.ll 
which crashes opt with SIGABRT — a pre-existing defect unrelated to this PR.

https://github.com/llvm/llvm-project/pull/213240