[DAG] SimplifyDemandedBits - relax "xor (X >> ShiftC), XorC --> (not X) >>...

[DAG] SimplifyDemandedBits - relax "xor (X >> ShiftC), XorC --> (not X) >> ShiftC" to match only demanded bits

The "xor (X >> ShiftC), XorC --> (not X) >> ShiftC" fold is currently limited to the XOR mask being a shifted all-bits mask, but we can relax this to only need to match under the demanded bits.

This helps expose more bit extraction/clearing patterns and fixes the PowerPC testCompares*.ll regressions from D127115

Alive2: https://alive2.llvm.org/ce/z/fl7T7K

Differential Revision: https://reviews.llvm.org/D129933