[Flang] Give fir.if RegionBranchOpInterface

fir.if currently isn't treated as a 'proper' conditional, so passes are unable to determine which regions are executed at times.

This patch gives fir.if this interface, which shouldn't do too much on its own but should allow future changes to take advantage
for various purposes

Reviewed By: vzakhari

Differential Revision: https://reviews.llvm.org/D145165