nir: Fix copy_prop_src when src is an indirect access on a reg.

nir: Fix copy_prop_src when src is an indirect access on a reg.

The intent was to continue down the indirect chain, not to call ourselves
with unchanged input arguments.  Found by code inspection, and comparison
to copy_prop_alu_src().

We haven't hit this because callers of NIR's copy prop are doing so in
SSA, before indirect variable dereferences have been lowered to registers.

Reviewed-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>