[PowerPC][altivec] Optimize codegen of vec_promote

According to https://www.ibm.com/docs/en/xl-c-and-cpp-linux/16.1.1?topic=functions-vec-promote, elements not specified by the input index argument are undefined. So that we don't need to set these elements to be zeros.

Reviewed By: nemanjai, #powerpc

Differential Revision: https://reviews.llvm.org/D158487