ff_fragment_shader: Don't generate swizzles for scalar combiner inputs

ff_fragment_shader: Don't generate swizzles for scalar combiner inputs

There are a couple scenarios where the source could be zero and the
operand could be either SRC_ALPHA or ONE_MINUS_SRC_ALPHA.  For
example, if the source was ZERO.  This would result in something like
(0).w, and a later call to ir_validate would get angry.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=42517