cmd/compile: optimize 386's assembly generator

The ADDconstmodify has similar logic with other constmodify like
instructions. This CL optimize them to share code via fallthrough.
And the size of pkg/linux_386/cmd/compile/internal/x86.a decreases
about 0.3KB.

Change-Id: Ibdf06228afde875e8fe8e30851b50ca2be513dd9
Reviewed-on: https://go-review.googlesource.com/136398


Run-TryBot: Ben Shi <powerman1st@163.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>