[IRCE][NFC] Make range check's End a non-null SCEV

Currently, IRC contains `Begin` and `Step` as SCEVs and `End` as value.
Aside from that, `End` can also be `nullptr` which can be later conditionally
converted into a non-null SCEV.

To make this logic more transparent, this patch makes `End` a SCEV and
calculates it early, so that it is never a null.

Differential Revision: https://reviews.llvm.org/D39590

llvm-svn: 322364