i965/fs: Optimize 16-bit SSBO stores by packing two into a 32-bit reg

i965/fs: Optimize 16-bit SSBO stores by packing two into a 32-bit reg

Currently, we use byte-scattered write messages for storing 16-bit
into an SSBO. This is because untyped surface messages have a fixed
32-bit size.

This patch optimizes these 16-bit writes by combining 2 values (e.g,
two consecutive components aligned with 32-bits) into a 32-bit register,
packing the two 16-bit words.

16-bit single component values will continue to use byte-scattered
write messages. The same will happens when the first consecutive
component is not aligned 32-bits.

This optimization reduces the number of SEND messages used for storing
16-bit values potentially by 2 or 4, which cuts down execution time
significantly because byte-scattered writes are an expensive
operation as they only write a component for message.

v2: Removed use of stride = 2 on sources (Jason Ekstrand)
    Rework optimization using shuffle 16 write and enable writes
    of 16bit vec4 with only one message of 32-bits. (Chema Casanova)
v3: - Fix coding style (Eduardo Lima)
    - Reorganize code to avoid duplication. (Jason Ekstrand)
    - Include new comments to explain the length calculations to
      fix alignment issues of components. (Jason Ekstrand)
    - Fix issues with writemask yz with 16-bit writes. (Jason Ektrand)
v4: (Jason Ekstrand)
    - Reorganize 64-bit ssbo-writes to avoid using slots_per_component.
    - Comment about why suffle is needed when using byte_scattered_write.

Signed-off-by: Eduardo Lima <elima@igalia.com>
Signed-off-by: Jose Maria Casanova Crespo <jmcasanova@igalia.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>