R600: Correctly set the src value offset for scalarized kernel args

This for some reason fixes v1i64 kernel arguments on pre-SI. This
currently breaks some other cases in the kernel-args.ll test for R600,
but I'm not particularly confident in the new output. VTX_READ_* are not
used for some of the scalarized cases, and the code reading from the
constant buffer doesn't make much sense to me.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215564 91177308-0d34-0410-b5e6-96231b3b80d8