mips specific changes for BZ 507033

mips specific changes for BZ 507033

Rework code to use Iop_ClzNat32 instead of the deprecated Iop_Clz32.
Likewise for Iop_Clz64.

For Iop_Clz32 a CLZ insn will be emitted which behaves naturally when
its input is 0. That is: CLZ(0) == 32.
Similarly, for Iop_Clz64 a DCLZ will be emitted with DCLZ(0) == 64.
That means we can replace Iop_Clz32/64 with Iop_ClzNat32/64 and remove
any IR that handles the input-is-zero case.
See also commit a5c48217e94.

Part of fixing https://bugs.kde.org/show_bug.cgi?id=507033