cmd/compile: fix type assert in dict pass

cmd/compile: fix type assert in dict pass

For type assertions, if src type is empty interface, we should
use normal type assertions rather than dynamic type assertions.

Fixes #53762

Change-Id: I596b2e4ad647fe5e42ad884f7273c78f8f50dac2
Reviewed-on: https://go-review.googlesource.com/c/go/+/416736
Run-TryBot: Wayne Zuo <wdvxdr@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>