cmd/compile: move all usage of delayTransform out of helpers.go

cmd/compile: move all usage of delayTransform out of helpers.go

So next CL will make delayTransform to become irgen's method, because
the delay transform logic also depends on irgen.topFuncIsGeneric field.

For #48609

Change-Id: I660ed19856bd06c3b6f4279a9184db96175dea2d
Reviewed-on: https://go-review.googlesource.com/c/go/+/351854
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>