Shared: Improve range analysis for phis at loop exits

[MathiasVP]

This adds support for inferring that i <= size once the loop exits in situations such as:

unsigned i = 0;
while(i < size) {
  i++;
}
use(i);

As @aschackmull noted this conclusion is only valid when we know that size >= 0 so for now it's restricted to the case where we can infer this constraint through the type of size. Ideally, this should be generalized later.