[CMake] Add new cmake option to control adding comments in GenDAGISel

Add new cmake option `LLVM_OMIT_DAGISEL_COMMENTS` to control adding
of comments in GenDAGISel for none debug builds

Ref: https://reviews.llvm.org/D78884

Reviewed By: nemanjai, MaskRay, #powerpc

Differential Revision: https://reviews.llvm.org/D114122