[dev.regabi] cmd/compile: remove okAs

[dev.regabi] cmd/compile: remove okAs

The check for blank in okAs is redundant with what its callers already
done, so just inline the conversion in callers side instead.

Passes toolstash-check.

Change-Id: I606105e2d2cf8e80214722a13c3101c464d20d82
Reviewed-on: https://go-review.googlesource.com/c/go/+/274793
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>