[PowerPC] Select the D-Form load if we know its offset meets the requirement

The LD/STD likewise instruction are selected only when the alignment in
the load/store >= 4 to deal with the case that the offset might not be
known(i.e. relocations). That means we have to select the X-Form load
for %0 = load i64, i64* %arrayidx, align 2 In fact, we can still select
the D-Form load if the offset is known. So, we only query the load/store
alignment when we don't know if the offset is a multiple of 4.

Reviewed By: jji, Nemanjai

Differential Revision: https://reviews.llvm.org/D93099