[globalisel][knownbits] Account for missing type constraints

Now that we look through copies, it's possible to visit registers that
have a register class constraint but not a type constraint. Avoid looking
through copies when this occurs as the SrcReg won't be able to determine
it's bit width or any known bits.

Along the same lines, if the initial query is on a register that doesn't
have a type constraint then the result is a default-constructed KnownBits,
that is, a 1-bit fully-unknown value.

llvm-svn: 371116