vc4: Try to pair up instructions when only one of them has PM bit

vc4: Try to pair up instructions when only one of them has PM bit

Instructions with difference in PM field can actually be paired up if
the one without PM doesn't do packing/unpacking and non-NOP
packing/unpacking operations from PM instruction aren't added to the
other without PM.

total instructions in shared programs: 48209 -> 47460 (-1.55%)
instructions in affected programs:     11688 -> 10939 (-6.41%)

Signed-off-by: Boyan Ding <boyan.j.ding@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>