cmd/gc: fix escape analysis for &x inside switch x := v.(type)

cmd/gc: fix escape analysis for &x inside switch x := v.(type)

The analysis for &x was using the loop depth on x set
during x's declaration. A type switch creates a list of
implicit declarations that were not getting initialized
with loop depths.

Fixes #8176.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/108860043