[InstCombine] fold sdiv with common shl amount in operands

(X << Z) / (Y << Z) --> X / Y

https://alive2.llvm.org/ce/z/CLKzqT

This requires a surprising "nuw" constraint because we have
to guard against immediate UB via signed-div overflow with
-1 divisor.

This extends 008a8903 and is another transform
derived from issue #58137.