cmd/compile: add store to load forwarding rules on riscv64

cmd/compile: add store to load forwarding rules on riscv64

Adds the equivalent integer variants of the floating point rules
added in CL 599235.

Change-Id: Ibe03c26383059821d99cea2337799e6416b4c581
Reviewed-on: https://go-review.googlesource.com/c/go/+/700175
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Julian Zhu <jz531210@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>