Don't eliminate wide store followed by a narrow one.

Don't eliminate wide store followed by a narrow one.

If the previous store operation is wider than the next one, we can't
eliminate it because there could be a subsequent wide load reading its
non-overwritten data.

Bug swiftshader:48

Change-Id: Iccb9cabad8ab8503140061ec4ce711a2959de33d
Reviewed-on: https://swiftshader-review.googlesource.com/10948
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>