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.