[TTI] [AArch64] getMemoryOpCost for ptr types

Opaque ptr types have a size in bits of 0. The legalised type is an i64 or
vector of i64s, which do have a size. Because of this difference in size, target
hook getMemoryOpCost modelled stores of ptr types as extending/truncating
load/stores. Now we just check for opaque ptr types and return the legalised
cost. This makes stores of pointers cheaper, and as a result we now SLP
vectorise the changed test case.

Differential Revision: https://reviews.llvm.org/D140193