When we moved racewalk to buildssa, we disabled SSA of structs with
2-4 fields while instrumenting because it caused false dependencies
because for "x.f" we would emit

    (StructSelect (Load (Addr x)) "f")

Even though we later simplify this to

    (Load (OffPtr (Addr x) "f"))

the instrumentation saw a load of x in its entirety and would issue
appropriate race/msan calls.

The fix taken here is to directly emit the OffPtr form when x.f is
addressable and can't be represented in SSA form.

Change-Id: I0caf37bced52e9c16937466b0ac8cab6d356e525
Reviewed-on: https://go-review.googlesource.com/109360
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>