cmd/compile: fix FMA negative commutativity of riscv64

cmd/compile: fix FMA negative commutativity of riscv64

According to RISCV manual 11.6:

FMADD x,y,z computes x*y+z and
FNMADD x,y,z => -x*y-z
FMSUB x,y,z => x*y-z
FNMSUB x,y,z => -x*y+z respectively

However our implement of SSA convert FMADD -x,y,z to FNMADD x,y,z which
is wrong and should be convert to FNMSUB according to manual.

Change-Id: Ib297bc83824e121fd7dda171ed56ea9694a4e575
Reviewed-on: https://go-review.googlesource.com/c/go/+/506575
Run-TryBot: M Zhuo <mzh@golangcn.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Joedian Reid <joedian@golang.org>
Reviewed-by: Michael Munday <mike.munday@lowrisc.org>
TryBot-Result: Gopher Robot <gobot@golang.org>