[llvm][Uniformity] provide overloads for Instruction* and Value*

Uniformity analysis is mainly concerned with the uniformity of values. But it is
sometimes useful to ask if an instruction is uniform, for example, if the
instruction is a terminator. On LLVM IR, every Instruction is a Value, so the
queries like isUniform() need to be overloaded so that the most derived class
always wins.

Reviewed By: ruiling

Differential Revision: https://reviews.llvm.org/D144699