[PowerPC] Change altivec indexed load/store builtins argument type

This patch changes the index argument of lvxl?/lve[bhw]x and
stvxl?/stve[bhw]x builtins from int to long. Because on 64-bit
subtargets, an extra extsw will always been generated, which is
incorrect.

Reviewed By: nemanjai

Differential Revision: https://reviews.llvm.org/D106530