cmd/compile: name change isDirect -> isDirectAndComparable

cmd/compile: name change isDirect -> isDirectAndComparable

Now that it also restricts to comparable types. Followon to CL 713840.

Change-Id: Idd975c3fd16fb51f55360f2fa0b89ab0bf1d00ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/715700
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Mateusz Poliwczak <mpoliwczak34@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>