[ARM] Use BaseT::getMemoryOpCost for getMemoryOpCost

This alters getMemoryOpCost to use the Base TargetTransformInfo version
that includes some additional checks for whether extending loads are
legal. This will generally have the effect of making <2 x ..> and some
<4 x ..> loads/stores more expensive, which in turn should help favour
larger vector factors.

Notably it alters the cost of a <4 x half>, which with the current
codegen will be expensive if it is not extended.

Differential Revision: https://reviews.llvm.org/D82456