cmd/compile: remove nodeNeedsWrapper flag

CL 254397 attached OVARLIVE nodes to OCALLxxx nodes Nbody.

The NeedsWrapper flag is now redundant with n.Nbody.Len() > 0
condition, so use that condition instead and remove the flag.

Passes toolstash-check.

Change-Id: Iebc3e674d3c0040a876ca4be05025943d2b4fb31
Reviewed-on: https://go-review.googlesource.com/c/go/+/254398


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