Reorder Value.def to optimize code size

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.

Differential Revision: https://reviews.llvm.org/D39373

llvm-svn: 318083