[InstSimplify] Fix a scalable-vector crash

D143505 fixed/simplified folding of operations with SNaN operands. In
doing so it introduced a crash when handling scalable vector types,
wherein the scalable-vector ConstantVector was cast to a ConstantFP.

Since we know by that point in the code that if we've found a NaN, we're
dealing with a scalable-vector splat (as there are no other kinds of
scalable-vector constant for which that holds), we can grab the splatted
value and re-use the existing code, which will automatically splat the
new NaN back to a scalable vector for us.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D153566