MSL writer: make signed int overflow defined behaviour

Bug: tint:124
Change-Id: Icf545b633d6390ceb7f639e80111390005e311a1
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/60100
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: David Neto <dneto@google.com>

test/expressions/binary/left-shift/vector-vector/i32.wgsl.expected.wgsl

@@ -0,0 +1,6 @@
+[[stage(compute), workgroup_size(1)]]
+fn f() {
+  let a = vec3<i32>(1, 2, 3);
+  let b = vec3<u32>(4u, 5u, 6u);
+  let r : vec3<i32> = (a << b);
+}