[lld-macho][nfc] Put back shouldOmitFromOutput() asserts

I removed them in rG5de7467e982 but @thakis pointed out that
they were useful to keep, so here they are again. I've also converted
the `!isCoalescedWeak()` asserts into `!shouldOmitFromOutput()` asserts,
since the latter check subsumes the former.

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D104169