runtime: separate stack freeing from stack shrinking

Currently, shrinkstack will free the stack if the goroutine is dead.
There are only two places that call shrinkstack: scanstack, which will
never call it if the goroutine is dead; and markrootFreeGStacks, which
only calls it on dead goroutines.

Clean this up by separating stack freeing out of shrinkstack.

Change-Id: I7d7891e620550c32a2220833923a025704986681
Reviewed-on: https://go-review.googlesource.com/c/go/+/170890


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>