[flang] Add missing dependencies for flang/lib/Optimizer/Dialect

Patch D145640 wasn't building for me because .cpp files in
flang/lib/Optimizer/Dialect depend on the files generated in
.../build/include/llfm/IR.  But the .cpp file compilations were being
done before the needed include files were generated.

This patch fixes that by adding dependencies on "intrinsics_gen" to the
CMake files in two directories.

Differential Revision: https://reviews.llvm.org/D145825