The use of std::unordered_map was the main source of slowdowns in
the optimizer code, so it was re-written without any maps. In order
to do so, the information originally carried by the maps was moved
to user-defined information stored within Subzero classes. The
optimizer now manages the memory used to store this information.

Bug swiftshader:69
Bug b/67872293

Change-Id: I2757169f0d3d467766317af6e00e149b4317fb9c
Reviewed-on: https://swiftshader-review.googlesource.com/19508Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>