Properly check `target_features` not to trigger an assertion
Fixes#89875
I think it should be a condition instead of an assertion to check if it's a register as it's possible that `reg` is a register class.
Also, this isn't related to the issue directly, but `is_target_supported` doesn't check `target_features` attributes. Is there any way to check it on rustc_codegen_llvm?
r? `@Amanieu`
a9b2bfb5ed