Optimize range() to enable more auto-vectorization

[sklam]

It is known that Numba's range() implementation is not optimal for LLVM loop optimizers. Even when one manually unroll loop for a constant loop bound, the way range() is implemented prohibits the loop-vectorizer to compute the loop bounds; thus failing to auto-vectorize in many cases.

This patch focuses on adjusting the ind-var computation to avoid computing reminders and instead relies mainly on additions, multiplications and floordiv which are common in low-level address computations.

There is still a reminder computation only if user ask for length_of_iterator(iter(range())), but it is a rare use-case. LLVM is able to optimize-away that reminder (and the storage of range_iterator_type.count)

Here's a notebook with a use-case I used to optimize range(): https://gist.github.com/sklam/6beddb2041580ceea4f25e0e496f50a0/420bc173da75996a307465f134c23bb3f4bb6562.