PR #117350 made changes to the SLP vectorizer which introduced a
regression on some ARM benchmarks. Investigation narrowed it down to
suboptimal codegen for benchmarks that previously only used scalar (U/S)MLAL
instructions. The linked change meant the SLPVectorizer thought that
these could be vectorized. This change makes the cost of muls in
(U/S)MLAL patterns slightly cheaper to make sure scalar instructions are
preferred in these cases over SLP vectorization on targets supporting DSP
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