[SDAG] try to reduce compare of funnel shift equal 0

fshl (or X, Y), X, C ==/!= 0 --> or (shl Y, C), X ==/!= 0
fshl X, (or X, Y), C ==/!= 0 --> or (srl Y, BW-C), X ==/!= 0

This is similar to an existing setcc-of-rotate fold, but the
matching requires more checks for the more general funnel op:
https://alive2.llvm.org/ce/z/Ab2jDd

We are effectively decomposing the funnel shift into logical
shifts, reassociating, and removing a shift.

This should get us the final improvements for x86-64 that were
originally shown in D111530
( https://github.com/llvm/llvm-project/issues/49541 );
x86-32 still shows some SHLD/SHRD, so the pattern is not
matching there yet.

Differential Revision: https://reviews.llvm.org/D122919