Remove gc.root's findCustomSafePoints mechanism

Remove gc.root's findCustomSafePoints mechanism

Searching all of the existing gc.root implementations I'm aware of (all three of them), there was exactly one use of this mechanism, and that was to implement a performance improvement that should have been applied to the default lowering.

Having this function is requiring a dependency on a CodeGen class (MachineFunction), in a class which is otherwise completely independent of CodeGen. I could solve this differently, but given that I see absolutely no value in preserving this mechanism, I going to just get rid of it.

Note: Tis is the first time I'm intentionally breaking previously supported gc.root functionality. Given 3.6 has branched, I believe this is a good time to do this.

Differential Revision: http://reviews.llvm.org/D7004

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226305 91177308-0d34-0410-b5e6-96231b3b80d8