[mlir][scf] Add getNumRegionInvocations to IfOp

Implements the RegionBranchOpInterface method getNumRegionInvocations to `scf::IfOp` so that, when the condition is constant, the number of region executions can be analyzed by `NumberOfExecutions`.

Reviewed By: jpienaar, ftynse

Differential Revision: https://reviews.llvm.org/D115087