[ValueTracking] Don't special case wrapped ConstantRanges; NFCI

Use `getUnsignedMax` directly instead of special casing a wrapped
ConstantRange.

The previous code would have been "buggy" (and this would have been a
semantic change) if LLVM allowed !range metadata to denote full
ranges. E.g. in

  %val = load i1, i1* %ptr, !range !{i1 1, i1 1} ;; == full set

ValueTracking would conclude that the high bit (IOW the only bit) in
%val was zero.

Since !range metadata does not allow empty or full ranges, this change
is just a minor stylistic improvement.

llvm-svn: 251380