[VPlan] Preserve IsSingleScalar for hoisted predicated load. (#184453)
The predicated loads may be single scalar (e.g. for VF = 1). We should
preserve IsSingleScalar when hoisting them. As all loops access the same
address, IsSingleScalar must match across all loads in the group.
This fixes an assertion when interleaving-only with hoisted loads.
Fixes https://github.com/llvm/llvm-project/issues/184372
PR: https://github.com/llvm/llvm-project/pull/184453