Alias analysis: refactor for use by other driver loops. (#5380)

* Alias analysis: refactor for use by other driver loops.

This PR pulls the core of the alias analysis infrastructure into a
`process_inst()` method that operates on a single instruction, and
allows another compiler pass to apply store-to-load forwarding and
redundant-load elimination interleaved with other work. The existing
behavior remains unchanged; the pass's toplevel loop calls this
extracted method.

This refactor is a prerequisite for using the alias analysis as part of
a refactored egraph-based optimization framework.

* Review feedback: remove unneeded mut.

cranelift/codegen/src/context.rs

@@ -365,8 +365,8 @@ impl Context {
     /// by a store instruction to the same instruction (so-called
     /// "store-to-load forwarding").
     pub fn replace_redundant_loads(&mut self) -> CodegenResult<()> {
-        let mut analysis = AliasAnalysis::new(&mut self.func, &self.domtree);
-        analysis.compute_and_update_aliases();
+        let mut analysis = AliasAnalysis::new(&self.func, &self.domtree);
+        analysis.compute_and_update_aliases(&mut self.func);
         Ok(())
     }