MLBEDSW-7147: Enable weight buffering when opt for Size

- When optimizing for Size the scheduler does not try to add weight
buffering to the schedule since this would add extra SRAM usage to
the peak usage. However, for all other ops that uses less SRAM than
the peak there is memory available that could be used for weight
buffering and hence improve the performance.

- Removed limitation to only run optimize schedule when optimizing
for Performance. Regardless of optimizing for Performance or Size the
scheduler flow is the same except that the limit for max SRAM usage is
different.

Change-Id: I6880b35655e37b4916a9c15150f0b8e5126a1cd8
Signed-off-by: Johan Alfven <johan.alfven@arm.com>