`operands_match` was modifying instruction source operands in-place
(through the `brw_reg *src` pointer member) and relying on a
save/restore pattern to undo the modifications. Work on local copies
instead, which is simpler and avoids mutating shared state in a
comparison function.
Fixes: 47c4b38540 ("i965/fs: Allow CSE to handle MULs with negated arguments.")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/39814>