[SCEV] Compute max backedge count for loops with "shift ivs"

This teaches SCEV to compute //max// backedge taken counts for loops
like

    for (int i = k; i != 0; i >>>= 1)
      whatever();

SCEV yet cannot represent the exact backedge count for these loops, and
this patch does not change that.  This is really geared towards teaching
SCEV that loops like the above are *not* infinite.

llvm-svn: 251558