Fix handling of value aliases, and re-enable LICM.

Value aliases aren't instructions, so they don't have a location in the
CFG, so it's not meaningful to query whether a value alias is defined
within a loop.

lib/cretonne/src/context.rs

@@ -92,10 +92,8 @@ impl Context {
         self.legalize(isa)?;
         if isa.flags().opt_level() == OptLevel::Best {
             self.compute_domtree();
-            /* TODO: Re-enable LICM.
             self.compute_loop_analysis();
             self.licm(isa)?;
-            */
             self.simple_gvn(isa)?;
         }
         self.compute_domtree();