writer/hlsl: Use unsigned indices for UBOs

These indices were a mix of signed and unsigned.
Modulus on the signed integers was producing FXC warnings about performance.

Change-Id: Ib82f4296199a09d2f03be8b06314feefce0022e2
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/56765
Auto-Submit: Ben Clayton <bclayton@google.com>
Reviewed-by: David Neto <dneto@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

test/ptr_ref/load/local/ptr_uniform.wgsl.expected.hlsl

@@ -4,7 +4,7 @@ cbuffer cbuffer_v : register(b0, space0) {
 
 [numthreads(1, 1, 1)]
 void main() {
-  const int scalar_offset = (0u) / 4;
+  const uint scalar_offset = (0u) / 4;
   const int use = (asint(v[scalar_offset / 4][scalar_offset % 4]) + 1);
   return;
 }