[InstCombine] Handle const select arm in foldSelectCtlzToCttz()

The select arm that takes the ctlz result can also instead be a
constant with the bit width (as this is what the ctlz evaluates to
for a==0).

This avoids a regression when strengthening the
simplifyWithOpReplaced() fold.

Proof: https://alive2.llvm.org/ce/z/DMRL5A