cmd/compile: fix plan9-amd64 build

cmd/compile: fix plan9-amd64 build

The previous rules to combine indexed loads produced addresses like:

    From: obj.Addr{
        Type:   TYPE_MEM,
        Reg:    REG_CX,
        Name:   NAME_AUTO,
        Offset: 121,
        ...
    }

which are erroneous because NAME_AUTO implies a base register of
REG_SP, and cmd/internal/obj/x86 makes many assumptions to this
effect.  Note that previously we were also producing an extra "ADDQ
SP, CX" instruction, so indexing off of SP was already handled.

The approach taken by this CL to address the problem is to instead
produce addresses like:

    From: obj.Addr{
        Type:   TYPE_MEM,
        Reg:    REG_SP,
        Name:   NAME_AUTO,
        Offset: 121,
        Index:  REG_CX,
        Scale:  1,
    }

and to omit the "ADDQ SP, CX" instruction.

Downside to this approach is it requires adding a lot of new
MOV[WLQ]loadidx1 instructions that nearly duplicate functionality of
the existing MOV[WLQ]loadidx[248] instructions, but with a different
Scale.

Fixes #15001.

Change-Id: Iad9a1a41e5e2552f8d22e3ba975e4ea0862dffd2
Reviewed-on: https://go-review.googlesource.com/21245
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>