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/icall-simd.wat

@@ -0,0 +1,7 @@
+(module
+  (type $ft (func (param v128) (result v128)))
+  (func $foo (export "foo") (param i32) (param v128) (result v128)
+    (call_indirect (type $ft) (local.get 1) (local.get 0))
+  )
+  (table (;0;) 23 23 anyfunc)
+)