[X86] Emit a reg-reg copy for fast isel of vector bitcasts.

Previously we just updated a map and moved on. But it possible
we cached known bits information with the vreg that can be used by
another basic block. If the other basic block has a different view
of the VT these known bits won't make sense.

By emitting a copy we ensure we have different vregs before and
after the bitcast. This prevents the known bits from being used
with the wrong type.

Differential Revision: https://reviews.llvm.org/D82517