[X86][AMX][NFC] Remove assert for comparison between different BBs.

SmallSet may use operator `<` when we insert MIRef elements, so we
cannot limit the comparison between different BBs.

We allow MIRef() to be less that any initialized MIRef object, otherwise,
we always reture false when compare between different BBs.

Differential Revision: https://reviews.llvm.org/D101039