compiler/glsl: allow sequence op as a const expr in gles 1.0

compiler/glsl: allow sequence op as a const expr in gles 1.0

Allow the sequence operator to be a constant expression in GLSL ES
versions prior to GLSL ES 3.0

Fixes the following piglit test:
/all/spec/glsl-es-1.0/compiler/array-sized-by-sequence-in-parenthesis.vert

This is similar to the logic from process_initializer() which performs
the same check for constant variable initialization with sequence
operators.

v2: Fixed regression pointed out by Eduardo Lima Mitev

Signed-off-by: Lars Hamre <chemecse@gmail.com>
Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>