i965/fs: Simplify and fix buggy stride/offset calculations using subscript().

i965/fs: Simplify and fix buggy stride/offset calculations using subscript().

These were bashing the 'offset' and 'stride' values of several
registers without taking the previous value into account, which
probably didn't matter in practice for optimize_frontfacing_ternary()
because the 'tmp' register already had a known region, but it would
have given the wrong region as result in the other cases in
lower_integer_multiplication().  subscript(..., i) is a more
straightforward way to take the i-th field of a given type from each
channel of a register which should give the right answer as result
regardless of the original 'offset' and 'stride' parameters of the
register region.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>