intel/vec4: Reswizzle VF immediates too

Previously, an instruction like

mul(8) vgrf29.xy:F, vgrf25.yxxx:F, [-1F, 1F, 0F, 0F]

would get rewritten as

mul(8) vgrf0.yz:F, vgrf25.yyxx:F, [-1F, 1F, 0F, 0F]

The latter does not produce the correct result.  The VF immediate in the
second should be either [-1F, -1F, 1F, 1F] or [0F, -1F, 1F, 0F].  This
commit produces the former.

Fixes: 1ee1d8ab ("i965/vec4: Reswizzle sources when necessary.")
Reviewed-by: Matt Turner <mattst88@gmail.com>
(cherry picked from commit 47c2aa5b)