cmd/compile: handle aggregate OpArg in registers

cmd/compile: handle aggregate OpArg in registers

Also handles case where OpArg does not escape but has its address
taken.

May have exposed a lurking bug in 1.16 expandCalls,
if e.g., loading len(someArrayOfstructThing[0].secondStringField)
from a local.  Maybe.

For #40724.

Change-Id: I0298c4ad5d652b5e3d7ed6a62095d59e2d8819c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/293396
Trust: David Chase <drchase@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>