Minor perf improvements. (#1580)
* Minor perf improvements.
- Cache the vector sizes in IExecutionFrame and NodeIndexInfo to avoid calls to size().
- 2 instructions instead of 10
- Remove an unnecessary check in IExecutionFrame
- add a check to the ctor so we guarantee it's unnecessary
- Reserve memory for the vectors in BroadcastIterator
- saves reallocs if more than one value is added
- but rare with the mlperf models for multiple values to be added so benefit is limited.
- slight tweak to the Broadcaster ctor code to make it more readable
