cmd/compile: guard partially live arg spilling with number of registers

The code that does partially live in-register arg spilling is
currently guarded with GOEXPERIMENT=regabiargs. But on platforms
where GOEXPERIMENT=regabiargs is not enabled there are still tests
that use register args. Guard it with actual number of registers
used, so it covers both.

Should fix the freeBSD builder.

Change-Id: I0d3c49d7a2389096cb6b17ca35b9b4ce567bc91e
Reviewed-on: https://go-review.googlesource.com/c/go/+/311830


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>