24205ee860
[AltiVec](https://www.nxp.com/docs/en/reference-manual/ALTIVECPEM.pdf) doc says that it just uses the lower `log2(n)` bits of the shift-amount rather than the whole element-sized value. So there is no need to handle an overflow. Also adjusts 64-bit literals to utilize the explicit `UINT64_C` type.