[LV] Don't emit unused scalars for uniform instructions

If we identify an instruction as uniform after vectorization, we know that we
should only use the value corresponding to the first vector lane of each unroll
iteration. However, when scalarizing such instructions, we still produce values
for the other vector lanes. This patch prevents us from generating the unused
scalars.

Differential Revision: https://reviews.llvm.org/D24275

llvm-svn: 282087