Wasm: Add support for typed select instruction

Reference types introduces a typed select operation. It has identical execution
semantics so no codegen change is needed.

cranelift/wasmtests/select.wat

@@ -0,0 +1,19 @@
+(module
+  (func $untyped-select (result i32)
+  	i32.const 42
+  	i32.const 24
+  	i32.const 1
+  	select)
+
+  (func $typed-select-1 (result anyref)
+  	ref.null
+  	ref.null
+  	i32.const 1
+  	select (result anyref))
+
+  (func $typed-select-2 (param anyref) (result anyref)
+    ref.null
+    local.get 0
+    i32.const 1
+    select (result anyref))
+)