[TTI] Refine default cost for interleaved load groups with gaps

This patch refines the default cost for interleaved load groups having gaps. If
a load group has gaps, the legalized instructions corresponding to the unused
elements will be dead. Thus, we don't need to account for them in the cost
model. Instead, we only need to account for the fraction of legalized loads
that will actually be used.

Differential Revision: http://reviews.llvm.org/D20873

llvm-svn: 272385