peepmatic: Do not use paths in linear IR

Rather than using paths from the root instruction to the instruction we are
matching against or checking if it is constant or whatever, use temporary
variables. When we successfully match an instruction's opcode, we simultaneously
define these temporaries for the instruction's operands. This is similar to how
open-coding these matches in Rust would use `match` expressions with pattern
matching to bind the operands to variables at the same time.

This saves about 1.8% of instructions retired when Peepmatic is enabled.

cranelift/peepmatic/src/lib.rs

@@ -142,11 +142,10 @@ where
     sort_lexicographically(&mut opts);
 
     let automata = automatize(&opts);
-    let paths = opts.paths;
     let integers = opts.integers;
 
     if let Ok(path) = std::env::var("PEEPMATIC_DOT") {
-        let f = dot_fmt::PeepholeDotFmt(&paths, &integers);
+        let f = dot_fmt::PeepholeDotFmt(&integers);
         if let Err(e) = automata.write_dot_file(&f, &path) {
             panic!(
                 "failed to write GraphViz Dot file to PEEPMATIC_DOT={}; error: {}",
@@ -155,11 +154,7 @@ where
         }
     }
 
-    Ok(PeepholeOptimizations {
-        paths,
-        integers,
-        automata,
-    })
+    Ok(PeepholeOptimizations { integers, automata })
 }
 
 #[cfg(test)]