linker: Check that initializers for global variables match

This requires tracking a couple extra fields in ir_variable:

 * A flag to indicate that a variable had an initializer.

 * For non-const variables, a field to track the constant value of the
   variable's initializer.

For variables non-constant initalizers, ir_variable::has_initializer
will be true, but ir_variable::constant_initializer will be NULL.  The
linker can use the values of these fields to check adherence to the
GLSL 4.20 rules for shared global variables:

    "If a shared global has multiple initializers, the initializers
    must all be constant expressions, and they must all have the same
    value. Otherwise, a link error will result. (A shared global
    having only one initializer does not require that initializer to
    be a constant expression.)"

Previous to 4.20 the GLSL spec simply said that initializers must have
the same value.  In this case of non-constant initializers, this was
impossible to determine.  As a result, no vendor actually implemented
that behavior.  The 4.20 behavior matches the behavior of NVIDIA's
shipping implementations.

NOTE: This is candidate for the 7.11 branch.  This patch also needs
the preceding patch "glsl: Refactor generate_ARB_draw_buffers_variables
to use add_builtin_constant"

Signed-off-by: Ian Romanick <[email protected]>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=34687
Reviewed-by: Kenneth Graunke <[email protected]>
Acked-by: Paul Berry <[email protected]>

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