cmd/compile: simplify load+op rules

cmd/compile: simplify load+op rules

There's no need to use @block rules, as canMergeLoad makes sure that
the load and op are already in the same block.
With no @block needed, we also don't need to set the type explicitly.
It can just be inherited from the op being rewritten.

Noticed while working on #19284.

Change-Id: Ied8bcc8058260118ff7e166093112e29107bcb7e
Reviewed-on: https://go-review.googlesource.com/37585
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>