peepmatic: Make peepmatic optional to enable

Rather than outright replacing parts of our existing peephole optimizations
passes, this makes peepmatic an optional cargo feature that can be enabled. This
allows us to take a conservative approach with enabling peepmatic everywhere,
while also allowing us to get it in-tree and make it easier to collaborate on
improving it quickly.

cranelift/Cargo.toml

@@ -47,4 +47,5 @@ walkdir = "2.2"
 [features]
 default = ["disas", "wasm", "cranelift-codegen/all-arch"]
 disas = ["capstone"]
+enable-peepmatic = ["cranelift-codegen/enable-peepmatic", "cranelift-filetests/enable-peepmatic"]
 wasm = ["wat", "cranelift-wasm"]