[ET-VK] Minor performance improvements for buffer to int8 quantized packing.

This diff provides minor performance improvements for buffer to int8 quantized packing in the Vulkan runtime graph ops.

Differential Revision: [D74616519](https://our.internmc.facebook.com/intern/diff/D74616519/)

[ghstack-poisoned]

Author: trivedivivek

backends/vulkan/runtime/graph/ops/glsl/nchw_to_bitw8_image_nobitw8buffer.glsl

@@ -42,47 +42,25 @@ const lowp int packed_dim = unhash_packed_dim(t_layout);
  * Extends sign of int8
  */
 int extend_sign(int x) {
-  if (x >> 7 == 1) {
-    return x | 0xFFFFFF00;
-  }
-  return x;
+  return x | mix(0, 0xFFFFFF00, x >= (1 << 7));
 }
 
 ivec4 read_texel(ivec4 tidx) {
-  ivec4 tidx_to_use = tidx;
-  ivec4 sizes_to_use = sizes;
-  int packed_dim_to_use = packed_dim;
-  if (transpose_hw == 1) {
-    sizes_to_use.xy = sizes_to_use.yx;
-    tidx_to_use.xy = tidx.yx;
-
-    if (packed_dim == 1) {
-      packed_dim_to_use = 0;
-    }
-    if (packed_dim == 0) {
-      packed_dim_to_use = 1;
-    }
-  }
+  const ivec4 tidx_to_use = ivec4(mix(tidx.xy, tidx.yx, bvec2(transpose_hw == 1)), tidx.zw);
+  const ivec4 sizes_to_use = ivec4(mix(sizes.xy, sizes.yx, bvec2(transpose_hw == 1)), sizes.zw);
+  const int packed_dim_to_use = mix(packed_dim, packed_dim ^ transpose_hw, packed_dim < 2);
 
   const ivec4 buf_indices = tidx_to_nchwi(
       tidx_to_use, sizes_to_use, packed_dim_to_use);
 
-  int shift = (1 << 8) - 1;
-  ivec4 masks;
-  // Masks used to unpack 4x 8-bit values from a 32 bit integer. Note that
-  // little endian is assumed, as most processors use little endian. Thus the
-  // most significant bytes correspond to the "latter" packed values.
-  masks.x = shift << (8 * (buf_indices.x % 4));
-  masks.y = shift << (8 * (buf_indices.y % 4));
-  masks.z = shift << (8 * (buf_indices.z % 4));
-  masks.w = shift << (8 * (buf_indices.w % 4));
+  const int mask = (1 << 8) - 1;
 
   ivec4 out_tex = ivec4(0);
 
   [[unroll]] for (int i = 0; i < 4; ++i) {
     if (tidx[packed_dim] + i < sizes[packed_dim]) {
-      int in_texel = nchw_in[buf_indices[i] / 4];
-      int extracted_val = (in_texel & masks[i]) >> (8 * (buf_indices[i] % 4));
+      const int in_texel = nchw_in[buf_indices[i] >> 2];
+      int extracted_val = (in_texel >> (8 * (buf_indices[i] & 3))) & mask;
       extracted_val = extend_sign(extracted_val);
       out_tex[i] = extracted_val;
     }