Second installment of "BasicBlock operands to the back"

changes.

For InvokeInst now all arguments begin at op_begin().
The Callee, Cont and Fail are now faster to get by
access relative to op_end().

This patch introduces some temporary uglyness in CallSite.
Next I'll bring CallInst up to a similar scheme and then
the uglyness will magically vanish.

This patch also exposes all the reliance of the libraries
on InvokeInst's operand ordering. I am thinking of taking
care of that too.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@66920 91177308-0d34-0410-b5e6-96231b3b80d8