cmd/compile: don't insert VarDef for already-initialized results

Currently, when we about to emit code that sets the function
results and returns, it emits a VarDef. But in some cases, the
result node is actually live and holding useful data. VarDef
means that we are about to (re)initialize it so all previous
data are dead, but that is not true. Don't insert that.

Also don't add VarDef for register results. We are not going to
store anything (currently it doesn't cause problem, just
unnecessary).

Change-Id: I9dd3b70b4a3f5035af028b143fde8fafa2f11fa0
Reviewed-on: https://go-review.googlesource.com/c/go/+/310589


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>