cmd/compile: refactor out an almost-superfluous arg

Moved all "target" information into "storeRC"; it was a register
cursor, now it is a register cursor that also carries the store
target with it if there are no registers.  Also allows booby-trapping
to ensure that the target is unambiguously one or the other.

For #40724.

Change-Id: I53ba4b91679e5fcc89c63b7d31225135299c6ec6
Reviewed-on: https://go-review.googlesource.com/c/go/+/293397


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