Check the type of the array operand and the index operand before doing
other checks.  This simplifies the code a bit now (eliminating the
error_emitted parameter), and enables some later functional changes.

The shader

uniform float x[6];
uniform sampler2D s;
void main() { gl_Position.x = xx[s + 1]; }

still generates (only) the two expected errors:

0:3(33): error: `xx' undeclared
0:3(39): error: Operands to arithmetic operators must be numeric

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>