[flang][NFC] move loadIfRef to FIRBuilder

This will be useful for OpenMP too.

I changed the definition slightly to use `fir::isa_ref_type` (which also
includes llvm pointers) because I think it reads better using the common
type helpers. There shouldn't be any llvm pointers in lowering so this
isn't a functional change.

Commit series for by-ref openmp reductions: 1/3