[Flang][OpenMP] Use fir.if instead of scf.if in lastprivate lowering

For finding the last iteration of a loop, or the last section an
if condition is generated. Using scf::if can cause some lowering
issues since the code contained inside it can have branches. Using
fir::if instead ensures that the fir::if is lowered into branches
along with any code contained inside that can generate branches.

Fixes #62458.

Reviewed By: NimishMishra

Differential Revision: https://reviews.llvm.org/D149547