[ARM] Adjust strd/ldrd codegen alignment requirements

In change https://reviews.llvm.org/D152790, it was discovered that the
alignment requirement calculation for LDRD/STRD codegen was suboptimal
and the calculation for volatile loads and stores was adjusted.

This change here adopts the calculation for the remaining non-volatile
occurances.

Differential Revision: https://reviews.llvm.org/D153800