i965/vec4: Fix null destination register in 3-source instructions

A recent commit (see below) triggered some cases where conditional
modifier propagation and dead code elimination would cause a MAD
instruction like the following to be generated:

    mad.l.f0  null, ...

Matt pointed out that fs_visitor::fixup_3src_null_dest() fixes cases
like this in the scalar backend.  This commit basically ports that code
to the vec4 backend.

NOTE: I have sent a couple tests to the piglit list that reproduce this
bug *without* the commit mentioned below.  This commit fixes those
tests.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Tested-by: Tapani Pälli <tapani.palli@intel.com>
Cc: mesa-stable@lists.freedesktop.org
Fixes: ee63933a ("nir: Distribute binary operations with constants into bcsel")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105704
(cherry picked from commit 91225cb3)