nir: Add a pass to lower local variable accesses to SSA values

nir: Add a pass to lower local variable accesses to SSA values

This pass analizes all of the load/store operations and, when a variable is
never aliased (potentially used by an indirect operation), it is lowered
directly to an SSA value.  This pass translates to SSA directly and does
not require any fixup by the original to-SSA pass.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>