nir: Fix uninitialized use of 'replacement'.

nir: Fix uninitialized use of 'replacement'.

For intrinsics we don't care about, just skip to the next loop iteration
and process the next instruction.  We don't want to execute the rest of
the code.

This was a bug in commit cdfc05ea6e8c87876cdbf588aa8e03d70f3da4bb.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>