nir: Don't chase nir_registers

Trying to use nir_ssa_scalar on a shader that's gone out-of-SSA is risky. If
there's a nir_register accessed anywhere in the expression tree, trying to chase
moves at the root causes an assertion failure:

   test_basic: ../src/compiler/nir/nir.c:2930: nir_alu_instr_is_copy: Assertion `instr->src[0].src.is_ssa' failed.

In case we hit a nir_register, stop chasing further (indeed, it would be invalid
to do so.) Go as far as we can and stop.

This makes nir_ssa_scalar safe to use in instruction selection for backends that
use nir_from_ssa (most of them).

Fixes test_basic.kernel_preprocessor_macros on Valhall.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
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