cmd/compile: don't reserve X15 for float sub/div any more

cmd/compile: don't reserve X15 for float sub/div any more

We used to reserve X15 to implement the 3-operand floating-point
sub/div ops with the 2-operand sub/div that 386/amd64 gives us.

Now that resultInArg0 is implemented, we no longer need to
reserve X15 (X7 on 386).

Fixes #15584

Change-Id: I978e6c0a35236e89641bfc027538cede66004e82
Reviewed-on: https://go-review.googlesource.com/28272
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>