[NFC] Promote willNotOverflow() / getStrengthenedNoWrapFlagsFromBinOp() from...

[NFC] Promote willNotOverflow() / getStrengthenedNoWrapFlagsFromBinOp() from IndVars into SCEV proper

We might want to use it when creating SCEV proper in createSCEV(),
now that we don't `forgetValue()` in `SimplifyIndvar::strengthenOverflowingOperation()`,
which might have caused us to loose some optimization potential.