[InstCombine] Combine trunc (lshr X, BW-1) to i1 --> icmp slt X, 0 (#142593) (#143846)
Fixes #142593, the issue was fixed using the suggestion on the ticket
itself.
Godbolt: https://godbolt.org/z/oW5b74jc4
alive2 proof: https://alive2.llvm.org/ce/z/QHnD7e