[mlir][ArmSVE] Add basic mask generation operations

[mlir][ArmSVE] Add basic mask generation operations

These `arm_sve.cmp` functions are needed to generate scalable vector
masks as long as scalable vectors are not part of the standard types.
Once in standard, these can be removed and `std.cmp` can be used
instead.

Differential Revision: https://reviews.llvm.org/D103473