i965/fs: Reduce restrictions on interference in register coalescing.

i965/fs: Reduce restrictions on interference in register coalescing.

We previously only allowed coalescing registers that interfere (i.e.,
whose live ranges overlap) if the destination register's live range was
entirely inside the source's live range. This is unnecessary -- we only
need to check for interfering writes in the intersection of their live
ranges.

total instructions in shared programs: 1639470 -> 1638453 (-0.06%)
instructions in affected programs:     84751 -> 83734 (-1.20%)

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>