cmd/compile: simplify assert{E,I}2I{,2} calling conventions

This CL rebases CL 273694 on top of master with @mdempsky's permission.

For assertE2I and assertI2I, there's no need to pass through the
interface's data pointer: it's always going to come back unmodified.

For assertE2I2 and assertI2I2, there's no need for an extra bool
result parameter: it's redundant with testing the returned interface
value for nil.

Change-Id: Ic92d4409ad381952f875d3d74b8cf11c32702fa6
Reviewed-on: https://go-review.googlesource.com/c/go/+/292892


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>