Remove common subexpr in aom_flat_block_finder_run

Eliminate the common subexpression block[yi * block_size + xi] in the
code that calculates `mean` and `var` in aom_flat_block_finder_run().
This works around what looks like a MSVC 2022 compiler optimization
bug.

Bug: aomedia:3365
Change-Id: I2492eea7a149128bd641433b29ddf9a38b3bf2d4
(cherry picked from commit 002e191faa88da2f224a357e66dd8b81e4986d6b)

aom_dsp/noise_model.c

diff --git a/aom_dsp/noise_model.c b/aom_dsp/noise_model.c
index 70fdb7b..ed45087 100644
--- a/aom_dsp/noise_model.c
+++ b/aom_dsp/noise_model.c
@@ -610,8 +610,9 @@
           Gxy += gx * gy;
           Gyy += gy * gy;
 
-          mean += block[yi * block_size + xi];
-          var += block[yi * block_size + xi] * block[yi * block_size + xi];
+          const double value = block[yi * block_size + xi];
+          mean += value;
+          var += value * value;
         }
       }
       mean /= (block_size - 2) * (block_size - 2);