Scalable vectorization factors are printed as "vscale x VF" where VF is
the known minimum number of elements, a integer. Currently,
llvm-opt-report always expects a integer (like for vectorization with
fixed-sized vectors), and does not display any vectorization factor in
the output (just 'V', but without a number).
This patch adds support for scalable vectorization factors and prints
them as "VNx<VF>", so for example "VNx4". The "Nx" is used to
differentiate between fixed-sized and scalable factors, and is
consistent with the way LLVM mangles scalable vectors in other places.
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