Fix MergeConsecutiveStore for non-byte-sized memory accesses.

The bug showed up as a compile-time assertion failure:
  Assertion `NumBits >= MIN_INT_BITS && "bitwidth too small"' failed
when building msan tests on x86-64.

Prior to r236850, this bug was masked due to a bogus alignment check,
which also accidentally rejected non-byte-sized accesses. Afterwards,
an invalid ElementSizeBytes == 0 got further into the function, and
triggered the assertion failure.

It would probably be a good idea to allow it to handle merging stores
of unusual widths as well, but for now, to un-break it, I'm just
making the minimal fix.

Differential Revision: http://reviews.llvm.org/D9626

llvm-svn: 236927