cmd/compile/internal/walk: relocate a stray comment

The explanatory comment and the associated version counter variable
for the helper routine "wrapCall" seem to have been left behind in
walk.go during the big refactoring -- move it back to where it should
be, next to wrapCall in stmt.go. Also fix a small buglet in the
comment itself.

Change-Id: I8637a838214b216581be59e01149a72282a46526
Reviewed-on: https://go-review.googlesource.com/c/go/+/303729


Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>