[flang] fix optional pointer TARGET argument lowering in ASSOCIATED

The TARGET argument of ASSOCIATED has a special lowering to deal with
POINTER and ALLOCATABLE optional actual arguments because they may be
dynamically absent. The previous code was doing a ternary
(mlir::SelectOp) to deal with this case, but generated invalid
code for the unused argument (loading a nullptr fir.ref<fir.box>). This
was not detected until D133779 was merged and modified how fir.load are
lowered to LLVM for fir.box types.

Replace the select by a proper if to prevent the fir.load from being
reachable in context where it should not.

Differential Revision: https://reviews.llvm.org/D134174