[dev.typeparams] cmd/compile: directly set some simple expression types

This CL updates irgen to directly set the type for a bunch of basic
expressions that are easy to handle already. Trickier rewrites are
still handled with typecheck.Expr, but responsibility of calling that
is pushed down to the conversion of individual operations.

Change-Id: I774ac6ab4c72ad854860ab5c741867dd42a066b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/285058


Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>