[X86] In matchBitExtract, place all of the new nodes before Node's position in...

[X86] In matchBitExtract, place all of the new nodes before Node's position in the DAG for the topological sort.

We were using OrigNBits, but that put all the nodes before the node we used to start the control computation. This caused some node earlier than the sequence we inserted to be selected before the sequence we created. We want our new sequence to be selected first since it depends on OrigNBits.

I don't have a test case. Found by reviewing the code.

llvm-svn: 356979