DAG: Move get_dynamic_area_offset type check to IR verifier (#145268)

Also fix the LangRef to match the implementation. This was checking
against the alloca address space size rather than the default address
space.

The check was also more permissive than the LangRef. The error
check permitted any size less than the pointer size; follow the
stricter wording of the LangRef.

llvm/docs/LangRef.rst

@@ -14600,7 +14600,7 @@ Semantics:
       compile-time-known constant value.
 
       The return value type of :ref:`llvm.get.dynamic.area.offset <int_get_dynamic_area_offset>`
-      must match the target's default address space's (address space 0) pointer type.
+      must match the target's :ref:`alloca address space <alloca_addrspace>` type.
 
 '``llvm.prefetch``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^