cmd/compile/internal/types2: rename is_X predicates back to isX (step 2 of 2)

This is s/is_/is/ throughout. No other changes.

Change-Id: I1be77a209133edc68a6dec0677a4991a7683f116
Reviewed-on: https://go-review.googlesource.com/c/go/+/361134


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>