Compiler: fix inferred nothrow effects for add_ptr and sub_ptr (#59720)
Previously, add_ptr and sub_ptr intrinsics were incorrectly inferred as
potentially throwing because they fell through to the general primitive
type check, which was incorrect after #53687 changed them. This adds
explicit handling in intrinsic_exct to return Union{} (nothrow) when
given correct argument types (Ptr and UInt), similar to #57398.
Fixes #57557
Written by Claude