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

Split SimpleConstantVal up into its components, so each Constant subclass getsa 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: 23467