nir/gather_info: Look for uses of helper invocations

The one obvious omission here is gl_HelperInvocation itself.  However,
the spec doesn't require that we generate then when gl_HelperInvocation
is used, it merely mandates that we report them if they are there.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/compiler/shader_info.h

@@ -188,6 +188,14 @@ typedef struct shader_info {
       struct {
          bool uses_discard;
 
+         /**
+          * True if this fragment shader requires helper invocations.  This
+          * can be caused by the use of ALU derivative ops, texture
+          * instructions which do implicit derivatives, and the use of quad
+          * subgroup operations.
+          */
+         bool needs_helper_invocations;
+
          /**
           * Whether any inputs are declared with the "sample" qualifier.
           */