cmd/compile: prevent shapifying of pointer shape type
CL 641955 changes the Unified IR reader to not doing shapify when
reading reshaping expression, prevent losing of the original type.
This is an oversight, as the main problem isn't about shaping during the
reshaping process itself, but about the specific case of shaping a
pointer shape type. This bug occurs when instantiating a generic
function within another generic function with a pointer shape type as
type parameter, which will convert `*[]go.shape.T` to `*go.shape.uint8`,
resulting in the loss of the original expression's type.
This commit changes Unified IR reader to avoid pointer shaping for
`*[]go.shape.T`, ensures that the original type is preserved when
processing reshaping expressions.