cmd/compile: allow SSA of multi-field structs while instrumenting
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.