R600/SI: Fix assertion from copying a TargetGlobalAddress

Assert in scheduler from an inserted copy_to_regclass from
a constant.

This only seems to break sometimes when a constant initializer
address is forced into VGPRs in a non-entry block. No test
since the only case I've managed to hit only happens with a future
patch, and that case will also not be a problem once scalar instructions
are used in non-entry blocks.

llvm-svn: 217380