[PowerPC] Vector load/store builtins overstate alignment of pointers

A number of builtins in altivec.h load/store vectors from pointers to scalar
types. Currently they just cast the pointer to a vector pointer, but expressions
like that have the alignment of the target type. Of course, the input pointer
did not have that alignment so this triggers UBSan (and rightly so).

This resolves https://bugs.llvm.org/show_bug.cgi?id=39704

Differential revision: https://reviews.llvm.org/D54787

llvm-svn: 347556