Vector element extraction without stack operations on Power 8

This patch corresponds to review:
http://reviews.llvm.org/D12032

This patch builds onto the patch that provided scalar to vector conversions
without stack operations (D11471).
Included in this patch:

    - Vector element extraction for all vector types with constant element number
    - Vector element extraction for v16i8 and v8i16 with variable element number
    - Removal of some unnecessary COPY_TO_REGCLASS operations that ended up
      unnecessarily moving things around between registers

Not included in this patch (will be in upcoming patch):

    - Vector element extraction for v4i32, v4f32, v2i64 and v2f64 with
      variable element number
    - Vector element insertion for variable/constant element number

Testing is provided for all extractions. The extractions that are not
implemented yet are just placeholders.

llvm-svn: 249822