i965/vec4: consider subregister offset in live variables

i965/vec4: consider subregister offset in live variables

Take into account offset values less than a full register (32 bytes)
when getting the var from register.

This is required when dealing with an operation that writes half of the
register (like one d2x in IVB/BYT, which uses exec_size == 4).

v2:
- Take in account this offset < 32 in liveness analysis too (Curro)

v3:
- Change formula in var_from_reg() (Curro)
- Remove useless changes (Curro)

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>