[release-branch.go1.15] cmd/compile: make Haspointers a method instead of a function

[release-branch.go1.15] cmd/compile: make Haspointers a method instead of a function

More ergonomic that way. Also change Haspointers to HasPointers
while we are here.

Change-Id: I45bedc294c1a8c2bd01dc14bd04615ae77555375
Reviewed-on: https://go-review.googlesource.com/c/go/+/249959
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/255318
Trust: Keith Randall <khr@golang.org>
Reviewed-by: Austin Clements <austin@google.com>