[release-branch.go1.10] cmd/compile/internal/gc: OMUL should be evaluated when using soft-float

When using soft-float, OMUL might be rewritten to function call
so we should ensure it was evaluated first.

Updates #28688
Fixes #28959

Change-Id: I30b87501782fff62d35151f394a1c22b0d490c6c
Reviewed-on: https://go-review.googlesource.com/c/148837


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
(cherry picked from commit c92e73b7)
Reviewed-on: https://go-review.googlesource.com/c/151343


Reviewed-by: Filippo Valsorda <filippo@golang.org>