[LV] Enable the LoopVectorizer to create pointer inductions

This patch enables the LoopVectorizer to build a phi of pointer
type and provide the vector loads and stores with vector type
getelementptrs built from the pointer induction variable, which
produces much less instructions than the previous approach of
creating scalar getelementpointers and glue them together to a
vector.

Differential Revision: https://reviews.llvm.org/D81267