cmd/compile: fix the index variable is shadowed in dictPass

The CL 349613 causes this problem.
In fact, we want to use the outer i to find m.List[i],
but the newly created index variable i in the nearest
for range shadow the outer i.

Fixes #48838.

Change-Id: I10f0bd985340f9443eefaadda6fc56e4e7e9a10c
Reviewed-on: https://go-review.googlesource.com/c/go/+/354549


Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>
Trust: Dan Scales <danscales@google.com>