glsl: Check array size is const before asserting that no IR was generated.

glsl: Check array size is const before asserting that no IR was generated.

process_array_type() contains an assertion to verify that no IR
instructions are generated while processing the expression that
specifies the size of the array.  This assertion needs to happen
_after_ checking whether the expression is constant.  Otherwise we may
crash on an illegal shader rather than reporting an error.

Fixes piglit tests array-size-non-builtin-function.vert and
array-size-with-side-effect.vert.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>