[dev.unified] cmd/compile: start using runtime dictionaries

This CL switches unified IR to start using runtime dictionaries,
rather than pure stenciling. In particular, for each instantiated
function `F[T]`, it now:

1. Generates a global variable `F[T]-dict` of type `[N]uintptr`, with
all of the `*runtime._type` values needed by `F[T]`.

2. Generates a function `F[T]-shaped`, with an extra
`.dict *[N]uintptr` parameter and indexing into that parameter for
derived types. (N.B., this function is not yet actually using shape
types.)

3. Changes `F[T]` to instead be a wrapper function that calls
`F[T]-shaped` passing `&F[T]-dict` as the `.dict` parameter.

This is done in one pass to make sure the overall wiring is all
working (especially, function literals and inlining).

Subsequent CLs will write more information into `F[T]-dict` and update
`F[T]-shaped` to use it instead of relying on `T`-derived information
itself. Once that's done, `F[T]-shaped` can be changed to
`F[shapify(T)]` (e.g., `F[go.shape.int]`) and deduplicated.

Change-Id: I0e802a4d9934794e01a6bfc367820af893335155
Reviewed-on: https://go-review.googlesource.com/c/go/+/420416


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>