glsl: Check TCS barrier restrictions at ast_to_hir time, not link time.

We want to check prior to optimization - otherwise we might fail to
detect cases where barrier() is in control flow which is always taken
(and therefore gets optimized away).

We don't currently loop unroll if there are function calls inside;
otherwise we might have a problem detecting barrier() in loops that
get unrolled as well.

Tapani's switch handling code adds a loop around switch statements, so
even with the mess of if ladders, we'll properly reject it.

Enforcing these rules at compile time makes more sense more sense than
link time.  Doing it at ast-to-hir time (rather than as an IR pass)
allows us to emit an error message with proper line numbers.
(Otherwise, I would have preferred the IR pass...)

Fixes spec/arb_tessellation_shader/compiler/barrier-switch-always.tesc.

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by; Ian Romanick <[email protected]>

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