Split SimpleConstantVal up into its components, so each Constant subclass gets

a different enum value.  This allows 'classof' for these to be really simple,
not needing to call getType() anymore.

This speeds up isa/dyncast/etc for constants, and also makes them smaller.
For example, the text section of a release build of InstCombine.cpp shrinks
from 230037 bytes to 216363 bytes, a 6% reduction.

llvm-svn: 23466