nir/algebraic: Remove problematic "optimization"

This line is no longer relevant now that booleans are 1-bit, and in fact
causes issues (infinite progress loop between algebraic optimizations
and copy prop) with constant vector masks.

No shader-db changes on Intel platforms (Jason).

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>