cmd/compile: fix Wasm rule file name

The rule generator expects the rule file name matches the arch's
name defined in
https://go.googlesource.com/go/+/b1df8d6ffa2c4c5be567934bd44432fff8f3c4a7/src/cmd/compile/internal/ssa/gen/WASMOps.go#197

Rename the file to match. Also rename WASMOps.go for consistency.

Fixes #25282.

Change-Id: I35c4bb2659fe67650933eb0ebf95778974511385
Reviewed-on: https://go-review.googlesource.com/111975
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Richard Musiol <neelance@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>