LowLevelType: replace a reachable llvm_unreachable with assert

assert is more appropriate here and fixes
`runtime error: execution reached an unreachable program point` in a
-DLLVM_USE_SANITIZER=Undefined build (-fno-sanitize-recover=all causes
llc to exit instead of crash (report_fatal_error)) when testing
MachineVerifier/test_g_assert_[sz]ext.mir.