st/glsl_to_tgsi: disable on-the-fly peephole for 64-bit operations

st/glsl_to_tgsi: disable on-the-fly peephole for 64-bit operations

This optimization is incorrect with 64-bit operations, because the
channel-splitting logic in emit_asm ends up being applied twice to
the source operands.

A lucky coincidence of how the writemask test works resulted in this
optimization basically never being applied anyway. As far as I can tell,
the only case where it would (incorrectly) have been applied is something
like

    dvec2 d;
    float x = (float)d.y;

which nobody seems to have ever done. But the moral equivalent does occur
in one of the component layout piglit test.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Dave Airlie <airlied@redhat.com>