Blender Git Loki

Cleanup: Use simpler loops in weld modifier

In this commit I changed many loops to range-based for loops.
I also removed some of the redundant iterator variables, using
indexing inside the loop instead. Generally an optimizing compiler
should have no problem doing the smartest thing in that situation,
and this way makes it much easier to tell where data is coming from.

I only changed the loops I was confident about, so there is still more
that could be done in the future.

Differential Revision: https://developer.blender.org/D13637