Bitcasting at control flow exits (#1272)

* Bitcast vectors immediately before a return

* Bitcast vectors immediately before a block end

* Use helper function for bitcasting arguments

* Add FuncTranslationState::peekn_mut; allows mutating of peeked values

* Bitcast values in place, avoiding an allocation

Also, retrieves the correct EBB header types for bitcasting on Operator::End.

* Bitcast values of a function with no explicit Wasm return instruction

* Add Signature::return_types method

This eliminates some duplicate code and avoids extra `use`s of `Vec`.

* Add Signature::param_types method; only collect normal parameters in both this and Signature::return_types

* Move normal_args to Signature::num_normal_params method

This matches the organization of the other Signature::num_*_params methods.

* Bitcast values of Operator::Call and Operator::CallIndirect

* Add DataFlowGraph::ebb_param_types

* Bitcast values of Operator::Br and Operator::BrIf

* Bitcast values of Operator::BrTable

cranelift/codegen/src/ir/dfg.rs

@@ -14,6 +14,7 @@ use crate::isa::TargetIsa;
 use crate::packed_option::ReservedValue;
 use crate::write::write_operands;
 use crate::HashMap;
+use alloc::vec::Vec;
 use core::fmt;
 use core::iter;
 use core::mem;
@@ -776,6 +777,14 @@ impl DataFlowGraph {
         self.ebbs[ebb].params.as_slice(&self.value_lists)
     }
 
+    /// Get the types of the parameters on `ebb`.
+    pub fn ebb_param_types(&self, ebb: Ebb) -> Vec<Type> {
+        self.ebb_params(ebb)
+            .iter()
+            .map(|&v| self.value_type(v))
+            .collect()
+    }
+
     /// Append a parameter with type `ty` to `ebb`.
     pub fn append_ebb_param(&mut self, ebb: Ebb, ty: Type) -> Value {
         let param = self.values.next_key();