Since we no longer have contextual nodes to indicate small arguments,
this should be safe to do. For register arguments it does not matter and
for stack arguments it is handled in codegen (for macOS arm64).

With small args we could end up inserting a cast between the arg node
and a GT_RET_EXPR. We would not handle substituting this GT_RET_EXPR
correctly. This reverts some more of the changes from 3e65d68 to make
the scenario work again, by skipping over GT_RET_EXPR before creating
the inlinee arg nodes.

…ution-small-types