cmd/compile: make type switch case positions consistent

CL 228106 moved the position at which we
checked whether a type switch variable had a particular type
from the type switch to the case statement, but only for
single, concrete types. This is a better position,
so this change changes the rest.

Change-Id: I601d4a5c4a0d9400e7804b9f1e729af948349a8f
Reviewed-on: https://go-review.googlesource.com/c/go/+/228220


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>