InstCombine/Demanded: simplify srem case (NFC) (#110260)
The srem case of SimplifyDemandedUseBits partially duplicates
KnownBits::srem. It is guarded by a statement that takes the absolute
value of the RHS and checks whether it is a power of 2, but the abs()
call here useless, since an srem with a negative RHS is flipped into one
with a positive RHS, adjusting LHS appropriately. Stripping the abs call
allows us to call KnownBits::srem instead of partially duplicating it.
