docs: reorganize front page slightly

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173718 91177308-0d34-0410-b5e6-96231b3b80d8
Add a comment mentioning that InstructionSimplify routines do,
in fact, resolve undef uses.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173721 91177308-0d34-0410-b5e6-96231b3b80d8
Remove the AttributeWithIndex class.

The AttributeWithIndex class exposed the interior structure of the AttributeSet
class. That was gross. Remove it and all of the code that relied upon it.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173722 91177308-0d34-0410-b5e6-96231b3b80d8