R600: Expand sign extension of vectors.

Setting vector types to expand will result in scalarization on pre SI hw,
as those gpus don't have vector shifts either.
Expand also i32 vectors, this helps llvm make the correct decision
about scalarizing the vector ops.

v2: move setOperation() calls to R600ISelLowering.cpp.
    cleanup the SI code to make it obvious that this patch does is nop for SI

Patch by: Jan Vesely <jan.vesely@rutgers.edu>

llvm-svn: 206348