Bug: lean-mlir does not fully support all syntax for llvm.icmp

[AtticusKuhn]

lean-mlir does support the syntax

llvm.icmp.eq
llvm.icmp.sgt
llvm.icmp.ne
...

But it does not support the syntax

llvm.icmp "eq"

which is allowed by the llvm mlir standard. See
llvm.icmp (LLVM::ICmpOp).

Note that we're not going to do anything about this yet, but I think it is good to document this is writing for any later people who stumble across this.