check for fastness before merging in DAGCombiner::MergeConsecutiveStores()

Use and check the 'IsFast' optional parameter to TLI.allowsMemoryAccess() any time
we have a merged access candidate. Without this patch, we were generating unaligned 
16-byte (SSE) memops for x86 targets where those accesses are slow.

This change was mentioned in:
http://reviews.llvm.org/D10662 and
http://reviews.llvm.org/D10905

and will help solve PR21711.

Differential Revision: http://reviews.llvm.org/D12573

llvm-svn: 246771