cmd/compile: guard partially live arg spilling with number of registers
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 <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Run-TryBot: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

golang/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,cloudflare/go,cloudflare/go,nawawi/go,golang/go,CAFxX/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,nawawi/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go