cmd/compile/internal: unexport gc.Oconv

cmd/compile/internal: unexport gc.Oconv

Updates #15462

Semi automatic change with gofmt -r and hand fixups for callers outside
internal/gc.

All the uses of gc.Oconv outside cmd/compile/internal/gc were for the
Oconv(op, 0) form, which is already handled the Op.String method.

Replace the use of gc.Oconv(op, 0) with op itself, which will call
Op.String via the %v or %s verb. Unexport Oconv.

Change-Id: I84da2a2e4381b35f52efce427b2d6a3bccdf2526
Reviewed-on: https://go-review.googlesource.com/22496
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>