[mlir][SideEffects] Mark the CFG only terminator operations as NoSideEffect

These terminator operations don't really have any side effects, and this allows for more accurate side-effect analysis for region operations. For example, currently we can't detect like a loop.for or affine.for are dead because the affine.terminator is "side effecting".

Note: Marking as NoSideEffect doesn't mean that these operations can be opaquely erased.

Differential Revision: https://reviews.llvm.org/D75888