Optimizer::optimizeStoresInSingleBasicBlock() was taking an extremely long time due to
looping through all the instructions of a single basic block at every iteration. By
creating a much smaller list of only load/store operations the first time a single basic
block is encountered and reusing that list when the same block is encountered again,
this function now runs ~10X faster.

In my test case:
out/Default/cc_unittests --gtest_filter=ImageBackgroundFilter.BackgroundFilterRotated_GL
The sw::optimize function went from taking almost 16s to roughly 1.5s.

Bug b/67872293

Change-Id: I7e2e2aa8dc1bf2663988fb59b58d72d9ee986e36
Reviewed-on: https://swiftshader-review.googlesource.com/19408Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>