[DAG] SimplifyDemandedBits - simplify rotl/rotr to shl/srl

If we only demand bits from one half of a rotation pattern, see if we can simplify to a logical shift.

For the ARM rev16 patterns, I had to drop a fold to prevent srl(bswap()) -> rotr(bswap) -> srl(bswap) infinite loops. I've replaced this with an isel PatFrag which should do the same task.

https://alive2.llvm.org/ce/z/iroxki (rol -> shl by amt iff demanded bits has at least as many trailing zeros as the shift amount)

https://alive2.llvm.org/ce/z/4ez_U- (ror -> shl by revamt iff demanded bits has at least as many trailing zeros as the reverse shift amount)

https://alive2.llvm.org/ce/z/cD7dR- (ror -> lshr by amt iff demanded bits has at least as many leading zeros as the shift amount)

https://alive2.llvm.org/ce/z/_XGHtQ (rol -> lshr by revamt iff demanded bits has at least as many leading zeros as the reverse shift amount)

Differential Revision: https://reviews.llvm.org/D114354