glsl: Ignore redundant prototypes after a function's been defined.

Consider the following shader:

    vec4 f(vec4 v) { return v; }
    vec4 f(vec4 v);

The prototype exactly matches the signature of the earlier definition,
so there's absolutely no point in it.  However, it doesn't appear to
be illegal.  The GLSL 4.30 specification offers two relevant quotes:

"If a function name is declared twice with the same parameter types,
 then the return types and all qualifiers must also match, and it is the
 same function being declared."

"User-defined functions can have multiple declarations, but only one
 definition."

In this case the same function was declared twice, and there's only one
definition, which fits both pieces of text.  There doesn't appear to be
any text saying late prototypes are illegal, so presumably it's valid.

Unfortunately, it currently triggers an assertion failure:
ir_dereference_variable @ <p1> specifies undeclared variable `v' @ <p2>

When we process the second line, we look for an existing exact match so
we can enforce the one-definition rule.  We then leave sig set to that
existing function, and hit sig->replace_parameters(&hir_parameters),
unfortunately nuking our existing definition's parameters (which have
actual dereferences) with the prototype's bogus unused parameters.

Simply bailing out and ignoring such late prototypes is the safest
thing to do.

Fixes Piglit's late-proto.vert as well as 3DMark/Ice Storm for Android.

NOTE: This is a candidate for stable branches.
Signed-off-by: Kenneth Graunke <[email protected]>
Tested-by: Tapani Pälli <[email protected]>
Reviewed-by: Jordan Justen <[email protected]>
Reviewed-by: Chad Versace <[email protected]>
Reviewed-by: Ian Romanick <[email protected]>

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