nir: Add an array copy optimization

nir: Add an array copy optimization

This peephole optimization looks for a series of load/store_deref or
copy_deref instructions that copy an array from one variable to another
and turns it into a copy_deref that copies the entire array.  The
pattern it looks for is extremely specific but it's good enough to pick
up on the input array copies in DXVK and should also be able to pick up
the sequence generated by spirv_to_nir for a OpLoad of a large composite
followed by OpStore.  It can always be improved later if needed.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>