EVTs potentially contain a Type * that points into memory owned by an
LLVMContext. Storing them in a function scoped static means they may
outlive the LLVMContext they point to.
This std::set is used to unique single element VT lists containing a
single extended EVT. Single element VT list with a simple EVT are
uniqued by a separate cache indexed by the MVT::SimpleValueType enum. VT
lists with more than one element are uniqued by a FoldingSet owned by
the SelectionDAG object.
This patch moves the single element cache into SelectionDAG so that it
will be destroyed when SelectionDAG is destroyed.
Fixes#88233
1d3f9f8862