Clarify SCEV code which handle powers of 2

[Andrew Trick]

> On Dec 15, 2014, at 1:02 PM, Mark Heffernan <meheff at google.com> wrote:
> 
> 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
> <scev_clearup.patch>

Perfect. Thanks Mark.
-Andy