[SVE] Fix TypeSize related warnings with IR truncates of scalable vectors

In getCastInstrCost when the instruction is a truncate we were relying
upon the implicit TypeSize -> uint64_t cast when asking if a given type
has the same size as a legal integer. I've changed the code to only
ask the question if the type is fixed length.

I have also changed InstCombinerImpl::SimplifyDemandedUseBits to bail
out for now if the type is a scalable vector.

I've added the following new tests:

  Analysis/CostModel/AArch64/sve-trunc.ll
  Transforms/InstCombine/AArch64/sve-trunc.ll

for both of these fixes.

Differential revision: https://reviews.llvm.org/D86432