nir: Be more clear that the new lowering helpers work on SSA defs

For more general per-instruction passes we need a different prototype
and different semantics.  While consuming a SSA def is actually slightly
more awkward, it makes the API more clear.  In particular, it's obvious
that you can never use it for lowering things that don't produce SSA
defs.
13 jobs for !1422 with review/nir-lower-helper-rename-ssa in 12 minutes and 58 seconds (queued for 3 seconds)
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debian

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meson-arm64

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meson-armhf

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meson-clang

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meson-vulkan

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scons-llvm

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