[Hexagon] Use getTypeAllocSize to compute difference between objects

The code was using getTypeStoreSize to calculate the difference
between consecutive objects. The calculation was incorrect due
to padding that is added between consecutive objects. The
getTypeAllocSize includes the padding amount. For example,
if the type is [19 x i8], the difference between consecutive
objects is 32 bytes, not 19 bytes.

A second case for getTypeAllocSize is needed when computing
the pointer values for the vector accesses. The calculation needs
to account for the padding as well.

Differential Revision: https://reviews.llvm.org/D109403