[mlir] VectorToSCF bug in setAllocAtFunctionEntry fixed.

The function makes too strong assumption regarding parent FuncOp
which gets broken when FuncOp is first lowered to llvm function.
In this fix we generalize the assumption to allocation scope and
add assertion to produce user friendly message in case our assumption
is broken.

Differential Revision: https://reviews.llvm.org/D86086