[GlobalISel] Make sure G_ASSERT_ZEXT's src ends up with the same rc as dst

When replacing the dst reg with the src reg, we need to make sure that we
propagate the dst reg's register class through to the src.

Otherwise, we aren't meeting the requirements for G_ASSERT_ZEXT, and so the
verifier will fail.

Differential Revision: https://reviews.llvm.org/D95708