[AST] Don't assert instruction reads/writes memory (PR51333)

This function is well-defined for an instruction that doesn't access
memory (and thus trivially doesn't alias anything in the AST), so
drop the assert. We can end up with a readnone call here if we
originally created a MemoryDef for an indirect call, which was
later replaced with a direct readnone call.

Fixes https://github.com/llvm/llvm-project/issues/51333.

Differential Revision: https://reviews.llvm.org/D127947