Before this change, REACTOR_DEFAULT_OPT_LEVEL was set to "Default" by
default, which would always define the REACTOR_DEFAULT_OPT_LEVEL macro
to this value. Because of this, the Optimization constructor would
forcibly set the Level member to this macro value. As a result, any
value we would try to set for Optimization in code (e.g. via
Optimization::Edit for a Routine generation) would be ignored.

With this change, only if the user sets the REACTOR_DEFAULT_OPT_LEVEL
CMake arg explicitly will it be used to override values in code.

Bug: b/174031014
Change-Id: Ifd6c7fdc520d5b063f2a982d4d508b6eed593006
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/50709
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>