Remove forward decl of SmallVectorImpl now that we have the header

Follow up to r211749.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211754 91177308-0d34-0410-b5e6-96231b3b80d8
R600: Fix vector FMA

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211757 91177308-0d34-0410-b5e6-96231b3b80d8