R600/SI: Fix another case of illegal VGPR to SGPR copy

This fixes a crash in Unigine Tropics.

https://bugs.freedesktop.org/show_bug.cgi?id=68389

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189057 91177308-0d34-0410-b5e6-96231b3b80d8