This patch adds new pseudo-insns for no-op vector extractions.

These were previously modelled as simple move instructions, but the 
register allocator has unhelpful special handling for these that 
triggered spills to memory. Modelling them as a vec_select does the 
right thing in the register allocator, but now the instruction has to be 
explicitly empty.

This patch has already been committed to the OG12 branch.

Andrew