[InstCombine] Fold lshr/ashr(or(neg(x),x),bw-1) --> zext/sext(icmp_ne(x,0)) (PR50816)

Handle the missing fold reported in PR50816, which is a variant of the existing ashr(sub_nsw(X,Y),bw-1) --> sext(icmp_sgt(X,Y)) fold.

We also handle the lshr(or(neg(x),x),bw-1) --> zext(icmp_ne(x,0)) equivalent - https://alive2.llvm.org/ce/z/SnZmSj

We still allow multi uses of the neg(x) - as this is likely to let us further simplify other uses of the neg - but not multi uses of the or() which would increase instruction count.

Differential Revision: https://reviews.llvm.org/D105764