[SparsePropagation] Use SmallVector for work lists

This patch changes the work lists from std::vector to SmallVector, which
matches the SCCP implementation. This patch also updates some related comments.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315373 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed