Canonicalize GEP source element types from `%T` to `[sizeof(%T) x i8]`.
This is intended to flush out any remaining places that rely on GEP
element types, as part of the `ptradd` migration. The impact of this
change is expected to be fairly minimal (we might enable a few more
hoist/sink style folds that depend on equal GEP types).
6ecbc0c96e