[CIR] Upstream compare expression support

[andykaylor]

Overview

Upstream support for compare expressions and the cir.cmp operation.

Suggested minimal test case

bool f(int a, int b) {
  return a > b;
}

Existing incubator tests

clang/test/CIR/CodeGen/cmp.cpp

[andykaylor]

@mmha is going to work on this after bin ops.