glsl: Just access the ir_expression_operation strings table directly

glsl: Just access the ir_expression_operation strings table directly

The operator_string functions gave us some protection against a
malformed table.  Now that the table is generated from the same data
that generates the enum, this is not a concern.  Just cut out the middle
man.

   text	   data	    bss	    dec	    hex	filename
7531892	 273992	  28584	7834468	 778b64	i965_dri-64bit-before.so
7531828	 273992	  28584	7834404	 778b24	i965_dri-64bit-after.so

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>