nir/algebraic: Use a cache to avoid re-emitting structs

nir/algebraic: Use a cache to avoid re-emitting structs

This takes the stupid simplest and most reliable approach to reducing
redundancy that I could come up with:  Just use the struct declaration
as the cach key.  This cuts the size of the generated C file to about
half and takes about 50 KiB off the .data section.

size before (release build):

   text	   data	    bss	    dec	    hex	filename
5363833	 336880	  13584	5714297	 573179	_install/lib64/libvulkan_intel.so

size after (release build):

   text	   data	    bss	    dec	    hex	filename
5229017	 285264	  13584	5527865	 545939	_install/lib64/libvulkan_intel.so

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