[DAG] expandIntMINMAX - attempt to match existing SETCC node

As noticed on D144789, when we have pairs of min/max nodes we often end up with multiple comparisons which we could reuse with commuted select ops, so check to see if a suitable SETCC already exists. This also allowed us to remove a similar X86 peephole.

There are other getSETCC cases where we could safely reuse other CondCodes as well - I've been trying to think of how we could reuse this logic in SelectionDAG but haven't found anything that always works well.

An alternative would be to have a TLI callback that returns a preferred CondCode from a list of options, I've noticed this helped fpclamptosat tests on some other targets (MVE + WebAssembly), but other tests suffered.

Differential Revision: https://reviews.llvm.org/D145065