[codegen] Don't simplify an operation if it would result in unnecessary legalization;

Converting something like iadd.i64 on a 32-bits architecture into a
iadd_imm.i64 will result in the instruction being legalized back to an
iadd.i64 later on, creating unnecessary churn.

This commit implements avoid doing so, and changes the target ISA to a
64-bits platform for tests than ran into this, as well as making sure
this won't happen on 32-bits platforms.

cranelift/codegen/src/context.rs

@@ -243,7 +243,7 @@ impl Context {
 
     /// Perform pre-legalization rewrites on the function.
     pub fn preopt(&mut self, isa: &dyn TargetIsa) -> CodegenResult<()> {
-        do_preopt(&mut self.func, &mut self.cfg);
+        do_preopt(&mut self.func, &mut self.cfg, isa);
         self.verify_if(isa)?;
         Ok(())
     }