Clarify SCEV code which handle powers of 2

[Mark Heffernan]

Hi Andy,

As discussed in the patch for fixing PR21694, here's a little patch which
clarifies the HowFarToZero computation when the step is a positive power of
two.  Functionally this should be identical to the existing code except for
the case where Step is maximally negative (eg, INT_MIN).  We now punt in
that one corner case to make reasoning about the code easier.

Mark
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