Use a union to store the various possible kinds of function pointers, and
access the correct one based on the flags.
SSEOpHelper_table6 and SSEOpHelper_table7 right now only have one case,
but this would change with AVX's 3- and 4-argument operations. Use
unions there too, to keep the code more similar for the three tables.
Extracted from a patch by Paul Brook <paul@nowt.org>.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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