[InstCombine] Add transform `(icmp pred (shl {nsw and/or nuw} X, Y), C)` -> `(icmp pred X, C)`

[goldstein.w.n]

Summary

Three new transforms:

1. (icmp pred (shl nsw nuw X, Y), C) [if C <= 0] -> (icmp pred X, C)
   • ugt: https://alive2.llvm.org/ce/z/K_57J_
   • sgt: https://alive2.llvm.org/ce/z/BL8u_a
   • sge: https://alive2.llvm.org/ce/z/yZZVYz
   • uge: https://alive2.llvm.org/ce/z/R4jwwJ
   • ule: https://alive2.llvm.org/ce/z/-gbmth
   • sle: https://alive2.llvm.org/ce/z/ycZVsh
   • slt: https://alive2.llvm.org/ce/z/4MzHYm
   • sle: https://alive2.llvm.org/ce/z/fgNfex
   • ult: https://alive2.llvm.org/ce/z/cXfvH5
   • eq : https://alive2.llvm.org/ce/z/sZh_Ti
   • ne : https://alive2.llvm.org/ce/z/UrqSWA
2. (icmp eq/ne (shl {nsw|nuw} X, Y), 0) -> (icmp eq/ne X, 0)
   • eq+nsw: https://alive2.llvm.org/ce/z/aSJN6D
   • eq+nuw: https://alive2.llvm.org/ce/z/r2_-br
   • ne+nuw: https://alive2.llvm.org/ce/z/RkETtu
   • ne+nsw: https://alive2.llvm.org/ce/z/8iSfW3
3. (icmp slt (shl nsw X, Y), 0/1) -> (icmp pred X, 0/1) (icmp sgt (shl nsw X, Y), 0/-1) -> (icmp pred X, 0/-1)
   • slt: https://alive2.llvm.org/ce/z/eZYRan
   • sgt: https://alive2.llvm.org/ce/z/QQeP26
     
     Transform 3) is really sle/slt/sge/sgt with 0, but sle/sge canonicalize to slt/sgt respectively so its implemented as such.

[goldstein.w.n]

ping.

[goldstein.w.n]

ping.

[nikic]

LGTM

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

2191–2195

Can we drop this fold now?