cmd/compile: fix wrong dict param when getting dict type

CL 338129 added getDictionaryType to get the dictionary type from the
specified dict param, but still using the one in info.dictParam, which
is wrong.

Fixes #51413

Change-Id: Ie13460c1e5751c4c5fc44479a44f6eed8b3b06e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/391994


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>