[CVP] LVI: Use in-block values when checking value signedness domain

This has a huge positive impact on all the folds that use these helpers,
as it can be seen on vanilla test-suite + rawspeed + darktable:
correlated-value-propagation.NumSRems             +75.68% (+ 28)
correlated-value-propagation.NumAShrs             +63.87% (+198)
correlated-value-propagation.NumSDivs             +49.42% (+127)
correlated-value-propagation.NumSExt              + 8.85% (+593)
correlated-value-propagation.NumUDivURemsNarrowed	+ 8.65% (+34)

... while having pretty minimal compile-time impact:
https://llvm-compile-time-tracker.com/compare.php?from=e8c7f43e2c2c6f3581ec1c6489ec21ad9f98958a&to=4cd197711e58ee1b2faeee0c35eea54540185569&stat=instructions