gallivm: (trivial) fix copy/paste bug with big endian code

8bd67a35c50e68c21aed043de11e095c284d151a introduced using undefined variable
on big endian archs due to copy/paste bug.
(compile hack tested only)
diff --git a/src/gallium/auxiliary/gallivm/lp_bld_gather.c b/src/gallium/auxiliary/gallivm/lp_bld_gather.c
index 7654ba0..2d1fb65 100644
--- a/src/gallium/auxiliary/gallivm/lp_bld_gather.c
+++ b/src/gallium/auxiliary/gallivm/lp_bld_gather.c
@@ -227,11 +227,13 @@
           * with src_width >= 32 here?
           */
       } else {
+         LLVMTypeRef dst_elem_type = lp_build_vec_type(gallivm, dst_type);
+
          /*
           * Only valid if src_ptr_type is int type...
           */
-         res = LLVMBuildZExt(gallivm->builder, res,
-                             lp_build_vec_type(gallivm, dst_type), "");
+         res = LLVMBuildZExt(gallivm->builder, res, dst_elem_type, "");
+
          if (vector_justify) {
 #ifdef PIPE_ARCH_BIG_ENDIAN
          res = LLVMBuildShl(gallivm->builder, res,