[ValueTracking] Match select abs pattern when there's an sext involved

When checking a select to see if it matches an abs, allow the true/false values
to be a sign-extension of the comparison value instead of requiring that they're
directly the comparison value, as all the comparison cares about is the sign of
the value.

This fixes a regression due to r333702, where we were no longer generating ctlz
due to isKnownNonNegative failing to match such a pattern.

Differential Revision: https://reviews.llvm.org/D47631

llvm-svn: 333927