Fix alignment checks in MergeConsecutiveStores.

1) check whether the alignment of the memory is sufficient for the
*merged* store or load to be efficient.

Not doing so can result in some ridiculously poor code generation, if
merging creates a vector operation which must be aligned but isn't.

2) DON'T check that the alignment of each load/store is equal. If
you're merging 2 4-byte stores, the first *might* have 8-byte
alignment, but the second certainly will have 4-byte alignment. We do
want to allow those to be merged.

llvm-svn: 236850