[X86] In matchBitExtract, if we need to truncate the BEXTR make sure we put...

[X86] In matchBitExtract, if we need to truncate the BEXTR make sure we put the BEXTR at Node's position in the DAG for the topological sort.

We were using OrigNBits, but that doesn't guarantee that it will be selected before the nodes that make up X.

llvm-svn: 356978