If the first values in Value.def is the range of constant, then the code generated by `isa<Constant>` is smaller by one operation (basically, an add is removed). It turns out this small optimization reduces the size of the statically linked clang binary by 400ko on my laptop. The theoritical performance gain is non visible from my benchmarks, but the size dropdown is.
Mmmh, build failing due to a funny warning that illustrates the optimization, see http://lab.llvm.org:8011/builders/ubuntu-gcc7.1-werror/builds/3012/steps/build-unified-tree/logs/stdio. Update in r318087.