cmd/compile: use IsSlice and IsArray instead of checking Bound

cmd/compile: use IsSlice and IsArray instead of checking Bound

Changes generated by eg and manually checked.

Isfixedarray, Isslice, and many other
Type-related functions in subr.go should
either be deleted or moved to type.go.
Later, though; the game now is cleanup via encapsulation.

Passes toolstash -cmp.

Change-Id: I83dd8816f6263b74367d23c2719a08c362e330f9
Reviewed-on: https://go-review.googlesource.com/21303
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>