nir/tex_instr: Add a nir_tex_src struct and dynamically allocate the src array

nir/tex_instr: Add a nir_tex_src struct and dynamically allocate the src array

This solves a number of problems.  First is the ability to change the
number of sources that a texture instruction has.  Second, it solves the
delema that may occur if a texture instruction has more than 4 sources.

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