cmd/compile: clean up store rules to use store type, not argument type

cmd/compile: clean up store rules to use store type, not argument type

Argument type is dangerous because it may be thinner than the actual
store being issued.

Change-Id: Id19fbd8e6c41390a453994f897dd5048473136aa
Reviewed-on: https://go-review.googlesource.com/c/go/+/483438
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: David Chase <drchase@google.com>