parser: make let shift operators work with all types

vlib/compiler/tests/shift_test.v

@@ -0,0 +1,33 @@
+fn test_shift_operators() {
+
+	// check that shift works with all integer types 
+	// as the right-hand side operand
+	a := 1
+	b := 1024
+	i := 10
+
+	assert b == a << i8(i)
+	assert b == a << byte(i)
+	assert b == a << i16(i)
+	assert b == a << u16(i)
+	assert b == a << int(i)
+	assert b == a << u32(i)
+	assert b == a << i64(i)
+	assert b == a << u64(i)
+
+	assert a == b >> i8(i)
+	assert a == b >> byte(i)
+	assert a == b >> i16(i)
+	assert a == b >> u16(i)
+	assert a == b >> int(i)
+	assert a == b >> u32(i)
+	assert a == b >> i64(i)
+	assert a == b >> u64(i)
+
+	// check that shift operation result type is
+	// the same as the type of the left-hand side operand
+	mut c := u64(0)
+	d := u64(1)
+	c = d << i8(63)
+	assert c == 9223372036854775808
+}