[CodeGen] Specified minnum/maxnum signed zero semantics not respected

LangRef specifies that minnum/maxnum have ordered signed zero unless nsz is specified. However, the backend does not fully respect these semantics right now. Known issues are:

* The X86 expansion does not respect signed zero ordering.
* Libcall legalization does not respect signed zero ordering, as the fmin/fmax libcalls are not guaranteed to have it (and indeed do not have it on many important targets like x86_64-gnu).

Start by reading the LangRef semantics for minnum/maxnum and then inspect the X86 expansion and libcall legalization paths mentioned in the issue. Verify how signed zero is handled when nsz is absent, including on x86_64-gnu, and add regression coverage showing that the specified ordering is preserved.

Written by the indexing model from the issue text.