[GISel]: Don't assert when constraining RegisterOperands which are uses.

Currently we assert that only non target specific opcodes can have
missing RegisterClass constraints in the MCDesc. The backend can have
instructions with register operands but don't have RegisterClass
constraints (say using unknown_class) in which case the instruction
defining the register will constrain it.
Change the assert to only fire if a def has no regclass.

https://reviews.llvm.org/D43409

llvm-svn: 326142