[dev.typeparams] cmd/compile: fix TestUnifiedCompare

We need to start the capture walk from expr.Body, not expr, otherwise
in quirks mode we'll set all of the captured variables' positions to
expr.Body.Rbrace.

Change-Id: Ic93f2773ae3756c2ec88dac17b4e9fb5a0771734
Reviewed-on: https://go-review.googlesource.com/c/go/+/330889


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>