[FastIsel][AArch64] Fix previous commit r214844 (Don't perform sign-/zero-extension for function arguments that have already been sign-/zero-extended.)

The original code would fail for unsupported value types like i1, i8, and i16.
This fix changes the code to only create a sub-register copy for i64 value types
and all other types (i1/i8/i16/i32) just use the source register without any
modifications.

getRegClassFor() is now guarded by the i64 value type check, that guarantees
that we always request a register for a valid value type.