[SCEV] Unify getUnsignedRange and getSignedRange

[SCEV] Unify getUnsignedRange and getSignedRange

Summary:
This removes some duplicated code, and also helps optimization: e.g. in
the test case added, `%idx ULT 128` in `@x` is not currently optimized
to `true` by `-indvars` but will be, after this change.

The only functional change in ths commit is that for add recurrences,
ScalarEvolution::getRange will be more aggressive -- computing the
unsigned (resp. signed) range for a SCEVAddRecExpr will now look at the
NSW (resp. NUW) bits and check for signed (resp. unsigned) overflow.
This can be a strict improvement in some cases (such as the attached
test case), and should be no worse in other cases.

Reviewers: atrick, nlewycky

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8142

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231709 91177308-0d34-0410-b5e6-96231b3b80d8