[PowerPC][altivec] Correct modulo number of vec_promote on vector char

According to https://www.ibm.com/docs/en/xl-c-and-cpp-linux/16.1.1?topic=functions-vec-promote, the index should be input modulo the number of elements in the vector. When the type is `vector char`, the number of elements should be 16.

Reviewed By: qiucf

Differential Revision: https://reviews.llvm.org/D158484