DAG: Replace legal type check in EmitCopyFromReg (#177788)
It doesn't make sense that an illegal type would get here; a
CopyFromReg cannot be illegally typed. The only exception that
was hit here is in a handful of SystemZ inline assembly tests
for i128, which use untyped. They shouldn't; it should treat
v2i64 as legal instead. Just leave the untyped check for now.
