small tweak to the SlangNode class

[Janther]

Initially this PR had included a small check and early exit when the rest of the execution wouldn't alter anything, but after some profiling, I realised that this was more taxing than just keep the execution untouched.

This inspired me to change the approach of storing leading and trailing offsets.

The important information we need to check is start - leadingOffset and end + trailingOffset. if we store this info into the appropriately named attributes outerStart and outerEnd, then we don't need to recalculate it at each call of updateMetadata. Also, we don't need to reassign leadingOffset and trailingOffset since the outer bounds are static.

This approach was profiled and there is a small improvement in the performance. I also appreciate the improvement in the simplicity of the code.