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/ir_validate.cpp

@@ -183,10 +183,14 @@ ir_validate::visit_leave(ir_expression *ir)
    case ir_unop_rcp:
    case ir_unop_rsq:
    case ir_unop_sqrt:
+      assert(ir->type == ir->operands[0]->type);
+      break;
+
    case ir_unop_exp:
    case ir_unop_log:
    case ir_unop_exp2:
    case ir_unop_log2:
+      assert(ir->operands[0]->type->base_type == GLSL_TYPE_FLOAT);
       assert(ir->type == ir->operands[0]->type);
       break;