cmd/compile: move some generic rules to strongly typed

Move a lot of the constant folding rules to use strongly
typed AuxInt fields.

We need more than a cast to convert AuxInt to, e.g., float32.
Make conversion functions for converting back and forth.

Change-Id: Ia3d95ee3583ee2179a10938e20210a7617358c88
Reviewed-on: https://go-review.googlesource.com/c/go/+/227866


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Giovanni Bajo <rasky@develer.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>