[dev.typeparams] cmd/compile: fold reader checking type params logic to separate...

[dev.typeparams] cmd/compile: fold reader checking type params logic to separate method

So making it less verbose and clearer to the reader what that check means.

Change-Id: I41587aab399e63600356c5cecec64978048bed36
Reviewed-on: https://go-review.googlesource.com/c/go/+/329571
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>