Replace `self.end() == usize::MAX` and `self.end() + 1 > slice.len()`
with `self.end() >= slice.len()`. Same reasoning as previous commit.
Also consolidate the str panicking functions into function.
Add a `codegen-llvm` test to check the number of `icmp` instrucitons
generated for each `SliceIndex` method on the various range types. This
will be updated in the next commit when `SliceIndex::get` is optimized
for `RangeInclusive`.