vulkan: Preprocess FA mask to detect all-neg-inf and all-zero. (#19281)

Write out a 2-bit code per block and avoid loading the mask when it
matches these two common cases.

Apply this optimization when the mask is relatively large (i.e. prompt
processing).

ggml/src/ggml-vulkan/vulkan-shaders/vulkan-shaders-gen.cpp

@@ -790,6 +790,8 @@ void process_shaders() {
     string_to_spv("split_k_reduce", "mul_mat_split_k_reduce.comp", {});
     string_to_spv("fa_split_k_reduce", "flash_attn_split_k_reduce.comp", {});
 
+    string_to_spv("fa_mask_opt", "flash_attn_mask_opt.comp", {});
+
     string_to_spv("quantize_q8_1", "quantize_q8_1.comp", {});
     string_to_spv("quantize_q8_1_subgroup", "quantize_q8_1.comp", {{"USE_SUBGROUPS", "1"}});