glsl2: Add a pass to convert exp and log to exp2 and log2.

Fixes ir_to_mesa handling of unop_log, which used the weird ARB_vp LOG
opcode that doesn't do what we want.  This also lets the multiplication
coefficients in there get constant-folded, possibly.

Fixes:
glsl-fs-log

src/glsl/linker.cpp

@@ -1281,6 +1281,7 @@ link_shaders(struct gl_shader_program *prog)
       do_mat_op_to_vec(ir);
       do_mod_to_fract(ir);
       do_div_to_mul_rcp(ir);
+      do_explog_to_explog2(ir);
 
       do {
 	 progress = false;