i965/fs: optimize pack double

When we are actually creating a double using values obtained from a
previous unpack operation we can bypass the unpack and source from
the original double value directly.

v2:
- Style changes (Topi)
- Bail is parent instruction's src is not SSA (Connor)

v3: Use subscript() instead of stride() (Curro)

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