[dev.typeparams] cmd/compile: remove SetClosureCalled(false) hacks

The current go/defer wrapping code goes to some length to clear
ClosureCalled when a function call will end up not being called
directly, and so it will need to use the context register.

But we already have a flag to indicate we need to use the context
register: Needctxt. The real issue here is just that buildssa was
using fn.ClosureCalled instead of fn.Needctxt.

Change-Id: Ic9f5f23b66eb467fc61fa84eacb45d46c54133d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/330329


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>