[GlobalISel] Reduce KnownBits usage in matcher combines (#92381)

Two icmp/and combines forced computation of KnownBits on all operands
everytime. We can avoid computing KnownBits on the LHS by exploiting a
couple of properties:
- Constants are always on the RHS for those instructions. If we have no
KnownBits on the RHS, we can bail out early and avoid computing LHS
knownbits.
- For icmp uge/ult 0, we don't need to know the KBs of the LHS to infer
the result

This allows to save some KnownBits calls, which are very expensive,
without affecting codegen.

0 files changed, 0 insertions, 0 deletions