[InstCombine] Avoid crash on aggregate types in SimplifyDemandedUseFPClass (#111128)
The disables folding for FP aggregates that are not poison/posZero
types, which is currently not supported. Note: To fully handle this
aggregates would also likely require teaching `computeKnownFPClass()` to
handle array and struct constants (which does not seem implemented
outside of zero init).