Fix incorrect kill flags in fastisel.

Fix incorrect kill flags in fastisel.

If called twice in the same BB on the same constant, FastISel::fastEmit_ri_ was marking the materialized vreg as killed on each use, instead of only the last use.

Change this to only mark the last use as killed by making earlier uses check if the vreg is already used elsewhere.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236650 91177308-0d34-0410-b5e6-96231b3b80d8