* target-def.h (TARGET_SCALAR_MODE_SUPPORTED_P): New.
        * target.h (struct gcc_target): Add scalar_mode_supported_p.
        * targhooks.c (default_scalar_mode_supported_p): New.
        * targhooks.h (default_scalar_mode_supported_p): Declare.
        * doc/tm.texi (TARGET_SCALAR_MODE_SUPPORTED_P): Document.

        * c-common.c (handle_mode_attribute): Query scalar_mode_supported_p
        before attempting to create types.  Tidy.
        * expr.c (vector_mode_valid_p): Use scalar_mode_supported_p.

        * config/alpha/alpha.c (alpha_scalar_mode_supported_p): New.
        (TARGET_SCALAR_MODE_SUPPORTED_P): New.

From-SVN: r86593