cmd/compile: generalize struct load/store

cmd/compile: generalize struct load/store

The SSA backend currently only handle struct with up to 4 fields. Thus,
there are different operations corresponding to number fields of the
struct.

This CL generalizes these with just one OpStructMake, allow struct types
with arbitrary number of fields.

However, the ssa.MaxStruct is still kept as-is, and future CL will
increase this value to optimize large structs.

Updates #24416

Change-Id: I192ffbea881186693584476b5639394e79be45c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/611075
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: David Chase <drchase@google.com>