cmd/compile: remove unnecessary casts and types from riscv64 rules

cmd/compile: remove unnecessary casts and types from riscv64 rules

This change shouldn't have any impact on codegen. It removes some
unnecessary int8 and int64 casts from the riscv64 rewrite rules.

It also removes explicit typing where the types already match:
`(OldOp <t>) => (NewOp <t>)` is the same as `(OldOp) => (NewOp)`.

Change-Id: Ic02b65da8f548c8b9ad9ccb6627a03b7bf6f050f
Reviewed-on: https://go-review.googlesource.com/c/go/+/719220
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Commit-Queue: Junyang Shao <shaojunyang@google.com>