Don't generate FP ops with 1 operand in memory for 387.

Change-Id: I23b49dfa2a1e60c8778c920230e64785a3ddfbd1
Reviewed-on: https://go-review.googlesource.com/105035
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>