cmd/compile: fix stack growing algorithm

The current stack growing implementation looks not right.
Specially, the line runtime/stack.go#L1068 never gets executed,
which causes many unnecessary copystack calls.

This PR is trying to correct the implementation.
As I'm not familiar with the code, the fix is just a guess.

Change-Id: I0bea1148175fad34f74f19d455c240c94d3cb78b
GitHub-Last-Rev: 57205f91fe6f7cecbf0b7aad0d90d2f81270b1e8
GitHub-Pull-Request: golang/go#47010
Reviewed-on: https://go-review.googlesource.com/c/go/+/332229


Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>