[ValueTracking] Slight refactor to avoid unnecessary work; NFC

Two changes:
    1) Make some cases that conditionally returned unconditional.
    2) In cases of `Op0 != 0 || Op1 != 0` its better check `Op1` first
       as its more likely to be a constant due to canonicalization (so
       faster to get knownbits of).

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D149419