[SCCP] convert signed div/rem to unsigned for non-negative operands, 2nd try

The original commit ( fe1f3cfc ) was reverted because it could
crash / assert when trying to fold a value that was replaced
by a constant. In that case, there might not be an entry for the
constant in the solver yet.

This version adds a check for that possibility along with tests to
exercise that pattern (they used to crash).

Original commit message:
This extends the transform added with D81756 to handle div/rem opcodes.
For example:
https://alive2.llvm.org/ce/z/cX6za6

This replicates part of what CVP already does, but the motivating example
from issue #57472 demonstrates a phase ordering problem - we convert
branches to select before CVP runs and miss the transform.

Differential Revision: https://reviews.llvm.org/D133198