ac/nir: Fix store_scratch with a non-full writemask

By adding one more helper to ac_llvm_build, we can also easily keep
vector stores together.

Fixes the
tests/spec/glsl-1.30/execution/fs-large-local-array-vec4.shader_test
piglit test.

Fixes: 74470baebb ("ac/nir: Lower large indirect variables to scratch")
Reviewed-by: Marek Olšák <marek.olsak@amd.com

src/amd/common/ac_llvm_build.h

@@ -190,6 +190,13 @@ LLVMValueRef
 ac_build_gather_values(struct ac_llvm_context *ctx,
 		       LLVMValueRef *values,
 		       unsigned value_count);
+
+LLVMValueRef
+ac_extract_components(struct ac_llvm_context *ctx,
+		      LLVMValueRef value,
+		      unsigned start,
+		      unsigned channels);
+
 LLVMValueRef ac_build_expand_to_vec4(struct ac_llvm_context *ctx,
 				     LLVMValueRef value,
 				     unsigned num_channels);