New warning for expanded vector operations

[Artem Shinkarov <artyom dot shinkaroff at gmail dot com>]

Hi

Here is a patch to inform a programmer about the expanded vector operation.
Bootstrapped on x86-unknown-linux-gnu.

ChangeLog:

        * gcc/tree-vect-generic.c (expand_vector_piecewise): Adjust to
          produce the warning.
          (expand_vector_parallel): Adjust to produce the warning.
          (lower_vec_shuffle): Adjust to produce the warning.
        * gcc/common.opt: New warning Wvector-operation-expanded.
        * gcc/doc/invoke.texi: Document the wawning.


Ok?


Thanks,
Artem Shinkarov.

P.S. It is hard to write a reasonable testcase for the patch, because
one needs to guess which architecture would expand a given vector
operation. But the patch is trivial.

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