cmd/compile: pre-spill pointers in aggregate-typed register args

There's a problem in liveness, where liveness of any
part of an aggregate keeps the whole aggregate alive,
but the not-live parts don't get spilled.  The GC
can observe those live-but-not-spilled slots, which
can contain junk.

A better fix is to change liveness to work
pointer-by-pointer, but that is also a riskier,
trickier fix.

To avoid this, in the case of

(1) an aggregate input parameter
(2) containing pointers
(3) passed in registers

pre-spill the pointers.

Updates #40724.

Change-Id: I6beb8e0a353b1ae3c68c16072f56698061922c04
Reviewed-on: https://go-review.googlesource.com/c/go/+/307909


Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>