[DAGCombiner] Remove unnecessary check on the size of the type of the index of EXTRACT_SUBVECTOR.

The type system already requires that the number of vector elements must fit in 32-bits so an index should as well. Even if the type of the index were larger all we care about is that the constant index can fit in 64-bits so that we can call getZExtValue.

llvm-svn: 293413