Introduce a new canonicalisation pass that forces a small set of
exponential-dataflow-prone instructions (SelectInst plus the FShl
and FShr intrinsic patterns) to be stored into a dedicated local
variable. Without this, expanding their value into C expressions
during decompilation can blow up into exponential path enumeration
on real binaries.
This is a temporary workaround. A more principled split of
pathological dataflows is planned and will replace this pass.
Alessandro Di Federico