legalizer/split.rs: split_ebb_params: avoid pointless Vec allocations. (#1025)

This function is responsible for 2.2% of all heap allocation (calls) in CL.
This change avoids all of them in the (presumably) common case where none of
the parameters require splitting.  It also slightly reduces the compiler's
instruction count.

cranelift/codegen/src/legalizer/split.rs

@@ -131,19 +131,27 @@ fn split_any(
 }
 
 pub fn split_ebb_params(func: &mut ir::Function, cfg: &ControlFlowGraph, ebb: Ebb) {
-    let mut repairs = Vec::new();
     let pos = &mut FuncCursor::new(func).at_top(ebb);
+    let ebb_params = pos.func.dfg.ebb_params(ebb);
 
-    for (num, ebb_param) in pos
-        .func
-        .dfg
-        .ebb_params(ebb)
-        .to_vec()
-        .into_iter()
-        .enumerate()
+    // Add further splittable types here.
+    fn type_requires_splitting(ty: Type) -> bool {
+        ty == ir::types::I128
+    }
+
+    // A shortcut.  If none of the param types require splitting, exit now.  This helps because
+    // the loop below necessarily has to copy the ebb params into a new vector, so it's better to
+    // avoid doing so when possible.
+    if !ebb_params
+        .iter()
+        .any(|ebb_param| type_requires_splitting(pos.func.dfg.value_type(*ebb_param)))
     {
-        let ty = pos.func.dfg.value_type(ebb_param);
-        if ty != ir::types::I128 {
+        return;
+    }
+
+    let mut repairs = Vec::new();
+    for (num, ebb_param) in ebb_params.to_vec().into_iter().enumerate() {
+        if !type_requires_splitting(pos.func.dfg.value_type(ebb_param)) {
             continue;
         }