[dev.regabi] cmd/compile/internal/types: add IsScalar query method

Add method Type.IsScalar() method, which returns TRUE
for numeric and pointer-shaped types, false for composites
such as string/array/slice/struct.

Change-Id: Ie53c71c07c5b3fbae11b48addd172343dc6bf3fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/274857


Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Than McIntosh <thanm@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>