[RewriteStatepointsForGC] For some values (like gep's and bitcasts) it's cheaper...

[RewriteStatepointsForGC] For some values (like gep's and bitcasts) it's cheaper to clone them after statepoint than to emit proper relocates for them. This change implements this logic. There is alredy similar optimization in CodeGenPrepare, but doing so during RewriteStatepointsForGC allows to capture more opprtunities such as relocates in loops and longer instruction chains.

Differential Revision: http://reviews.llvm.org/D9774

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237701 91177308-0d34-0410-b5e6-96231b3b80d8