nir/algebraic: Only replace an instruction once

nir/algebraic: Only replace an instruction once

Without the break, it was possible that an instruction would match multiple
expressions.  If this happened, you could end up trying to replace it
multiple times and get a segfault.  This makes it so that, after a
successful replacement, it moves on to the next instruction.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>