nir/loop_unroll: Fix out-of-bounds access handling

nir/loop_unroll: Fix out-of-bounds access handling

The previous code was completely broken when it came to constructing the
undef values.  I'm not sure how it ever worked.  For the case of a copy
that reads an undefined value, we can just delete the copy because the
destination is a valid undefined value.  This saves us the effort of
trying to construct a value for an arbitrary copy_deref intrinsic.

Fixes: e8a8937a04 "nir: add partial loop unrolling support"
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>