Parse select instructions.

cranelift/tests/parser/tiny.cton

@@ -12,3 +12,9 @@ ebb0:
     v1 = iconst.i8 6
     v2 = ishl v0, v1
 }
+
+; Polymorphic istruction controlled by second operand.
+function select() {
+ebb0(vx0: i32, vx1:i32, vx2: b1):
+    v0 = select vx2, vx0, vx1
+}

cranelift/tests/parser/tiny.cton.ref

@@ -9,3 +9,8 @@ ebb0:
     v1 = iconst.i8 6
     v2 = ishl v0, v1
 }
+
+function select() {
+ebb0(vx0: i32, vx1: i32, vx2: b1):
+    v0 = select vx2, vx0, vx1
+}