[bindings/go] Add bindings to LLVMGet?CmpPredicate

Add bindings so that predicates on comparisons (icmp/fcmp) can be
inspected from IR.

Note: I considered adding Value.ICmpPredicate() etc. instead but
Value.IntPredicate() seemed easier to read and matches the name of the
returned type.

(This change was also pushed two commits ago but accidentally had the
wrong title and description.)

Revision: https://reviews.llvm.org/D53884
llvm-svn: 362893