The rules for extracting the interface data word don't leave
the result typed correctly. If I do i.([1]*int)[0], the result
should have type *int, not [1]*int. Using (IData x) for the result
keeps the typing of the original top-level Value.

I don't think this would ever cause a real codegen bug, bug fixing it
at least makes the typing shown in ssa.html more consistent.

Change-Id: I239d821c394e58347639387981b0510d13b2f7b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/204042
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>