[FastISel] Reuse register for bitcast that does not change MVT

The current FastISel code reuses the register for a bitcast that
doesn't change the IR type, but uses a reg-to-reg copy if it
changes the IR type without changing the MVT. However, we can
simply reuse the register in that case as well.

In particular, this avoids unnecessary reg-to-reg copies for pointer
bitcasts. This was found while inspecting O0 codegen differences
between typed and opaque pointers.

Differential Revision: https://reviews.llvm.org/D119432