[GISel]: Verify COPIES involving generic registers.

Add verification for copies involving generic registers if they are
compatible - ie if it is a generic copy, then the types are the
same, and if a COPY b/w generic and target virtual register, then
the sizes should be the same. Only checks if there are no sub registers
involved for now.

https://reviews.llvm.org/D37775

llvm-svn: 324696