glsl: Restrict initializers for global variables to constant expression in ES

glsl: Restrict initializers for global variables to constant expression in ES

v2: Combine this check with the existing const and uniform checks.  This
change depends on the previous patch (glsl: Only set
ir_variable::constant_value for const-decorated variables).

Fixes:

    ES2-CTS.shaders.negative.initialize
    ES3-CTS.shaders.negative.initialize

    spec/glsl-es-1.00/compiler/global-initializer/from-attribute.vert
    spec/glsl-es-1.00/compiler/global-initializer/from-uniform.vert
    spec/glsl-es-1.00/compiler/global-initializer/from-uniform.frag
    spec/glsl-es-1.00/compiler/global-initializer/from-global.vert
    spec/glsl-es-1.00/compiler/global-initializer/from-global.frag
    spec/glsl-es-1.00/compiler/global-initializer/from-varying.frag
    spec/glsl-es-3.00/compiler/global-initializer/from-uniform.vert
    spec/glsl-es-3.00/compiler/global-initializer/from-uniform.frag
    spec/glsl-es-3.00/compiler/global-initializer/from-in.vert
    spec/glsl-es-3.00/compiler/global-initializer/from-in.frag
    spec/glsl-es-3.00/compiler/global-initializer/from-global.vert
    spec/glsl-es-3.00/compiler/global-initializer/from-global.frag

Note: spec/glsl-es-3.00/compiler/global-initializer/from-sequence.*
still fail because the result of a sequence operator is still considered
to be a constant expression.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92304
Reviewed-by: Tapani Pälli <tapani.palli@intel.com> [v1]
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com> [v1]
Reviewed-by: Matt Turner <mattst88@gmail.com>
Cc: "10.6 11.0" <mesa-stable@lists.freedesktop.org>