Clean up some of LVI:

 * mergeIn now uses constant folding for constants that are provably not-equal.
 * sink some sanity checks from the get*() methods into the mark*() methods, to ensure that we never have a constant/notconstant ConstantInt
 * some textual cleanups, whitespace changes, removing "else" after return, that sort of thing.

llvm-svn: 121877