nir: Fix mod analysis of ishl to shift the recursive result

When considering ((x << y) % divisor), we recursed to calculate
mod = (x % (divisor << y)) but incorrectly returned mod directly,
rather than the correct value, (mod << y).

(Note that we require divisor to be a power-of-two.)

As an example of this going wrong, (x << 1) % 4 was returning (x % 2)
which is 0 or 1, but x << 1 is 2x, which is always an even number so
the result mod 4 can only be 0 or 2.

Unit test suggested by Caio Oliveira during review.

Fixes: 2255375c4d ("nir: add nir_mod_analysis & its tests")
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38918>

src/compiler/nir/nir_mod_analysis.c

@@ -132,7 +132,11 @@ nir_mod_analysis(nir_scalar val, nir_alu_type val_type, unsigned div, unsigned *
                return true;
             }
             nir_alu_type type0 = nir_alu_src_type(alu, 0);
-            return nir_mod_analysis(nir_alu_arg(alu, 0, val.comp), type0, div >> shift, mod);
+            if (!nir_mod_analysis(nir_alu_arg(alu, 0, val.comp), type0, div >> shift, mod))
+               return false;
+
+            *mod <<= shift;
+            return true;
          }
          break;
       }

src/compiler/nir/tests/mod_analysis_tests.cpp

@@ -331,3 +331,20 @@ TEST_F(nir_mod_analysis_test, const_shr_const_overflow)
       }
    }
 }
+
+TEST_F(nir_mod_analysis_test, const_shl_const)
+{
+   /* (const << const) % const_mod should always be known */
+   for (unsigned const_mod = 1; const_mod <= 1024; const_mod *= 2) {
+      for (unsigned i = 0; i < 50; ++i) {
+         for (unsigned j = 0; j < 6; ++j) {
+            nir_def *shl = nir_ishl(b, v[i], v[j]);
+
+            unsigned mod = INT32_MAX;
+
+            EXPECT_TRUE(nir_mod_analysis_comp0(shl, nir_type_uint, const_mod, &mod));
+            EXPECT_EQ(mod, (i << j) % const_mod);
+         }
+      }
+   }
+}