[mlir][llvm] Add icmp folder (#65343)

This revision adds a simple icmp folder that performs the following
folds to the LLVM dialect icmp op:
 - cmpi(eq/ne, x, x) -> true/false
 - cmpi(eq/ne, alloca, null) -> false/true
 - cmpi(eq/ne, null, alloca) -> cmpi(eq/ne, alloca, null)