vc4: Coalesce instructions using VPM reads into the VPM read.

vc4: Coalesce instructions using VPM reads into the VPM read.

This is done instead of copy propagating the VPM reads into the
instructions using them, because VPM reads have to stay in order.

shader-db results:
total instructions in shared programs: 78509 -> 78114 (-0.50%)
instructions in affected programs:     5203 -> 4808 (-7.59%)
total estimated cycles in shared programs: 234670 -> 234318 (-0.15%)
estimated cycles in affected programs:     5345 -> 4993 (-6.59%)

Signed-off-by: Varad Gautam <varadgautam@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Tested-by: Rhys Kidd <rhyskidd@gmail.com>