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/wasmtests/call-simd.wat

@@ -0,0 +1,14 @@
+(module
+  (func $main
+    (v128.const i32x4 1 2 3 4)
+    (v128.const i32x4 1 2 3 4)
+    (call $add)
+    drop
+  )
+  (func $add (param $a v128) (param $b v128) (result v128)
+    (local.get $a)
+    (local.get $b)
+    (i32x4.add)
+  )
+  (start $main)
+)