[ExpandLargeFpConvert] Scalarize vector types. (#86954)

expand-large-fp-convert cannot handle vector types.
If overly large vector element types survive into
isel, they will likely be scalarized there, but since
isel cannot handle scalar integer types of that size,
it will assert.

Handle vector types in expand-large-fp-convert by
scalarizing them and then expanding the scalar type
operation. For large vectors, this results in a
*massive* code expansion, but it's better than
asserting.