cmd/compile: fix line numbers for index panics

In the statement x = a[i], the index panic should appear to come from
the line number of the '['. Previous to this CL we sometimes used the
line number of the '=' instead.

Fixes #29504

Change-Id: Ie718fd303c1ac2aee33e88d52c9ba9bcf220dea1
Reviewed-on: https://go-review.googlesource.com/c/go/+/174617


Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>