i965/fs: Fix propagation of copies with strided source.

i965/fs: Fix propagation of copies with strided source.

This has likely been broken since we started propagating copies not
matching the offset of the instruction exactly
(1728e74957a62b1b4b9fbb62a7de2c12b77c8a75).  The copy source stride
needs to be taken into account to find out the offset at the origin
that corresponds to the offset at the destination of the copy which is
being read by the instruction.  This has led to program miscompilation
on both my SIMD32 branch and Igalia's FP64 branch.

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