i965/fs: Don't CSE render target messages with different target index.

i965/fs: Don't CSE render target messages with different target index.

We weren't checking the fs_inst::target field when comparing whether
two instructions are equal.  For FB writes it doesn't matter because
they aren't CSE-able anyway, but this would have become a problem with
FB reads which are expression-like instructions.

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