Carry facts about nullness and undef across GC relocation

This change implements four basic optimizations:

    If a relocated value isn't used, it doesn't need to be relocated.
    If the value being relocated is null, relocation doesn't change that. (Technically, this might be collector specific. I don't know of one which it doesn't work for though.)
    If the value being relocated is undef, the relocation is meaningless.
    If the value being relocated was known nonnull, the relocated pointer also isn't null. (Since it points to the same source language object.)

I outlined other planned work in comments.

Differential Revision: http://reviews.llvm.org/D6600



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224968 91177308-0d34-0410-b5e6-96231b3b80d8
Semantic tests for memory invalidation at statepoints

These are simply a collection of tests intended to show that information about the contents of gc references in the heap is lost at a statepoint. I've tried to write them so that they don't disallow correct transformations, while still being fairly easy to understand.

p.s. Ideas for additional tests are welcome.

Differential Revision: http://reviews.llvm.org/D6491



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224971 91177308-0d34-0410-b5e6-96231b3b80d8