[Codegen] Use overflow-checked math in the CPU and GPU alloc size checks (#24745)

`checkStackAllocationSize` accumulates the allocation size in a plain
int64_t. Allocas sized by the upper bound of an unbounded dynamic
dimension (the util.assume.int default, 2^53-1) overflow it, the wrapped
value passes the limit check and the alloca reaches ConvertToLLVM, where
its element count folds to llvm.mlir.poison and the program loads and
stores through a garbage pointer at runtime.

The reproduction was reduced from the quantized ResNet downsample
construct: an ONNX model with a dynamic batch doing DequantizeLinear ->
1x1 stride-2 Conv compiles without diagnostics and returns completely
wrong results.

Move `getStaticShapeSizeInBits` to Codegen/Utils (taken from
https://github.com/iree-org/iree/pull/24684) and use it from both
LLVMCPUCheckIRBeforeLLVMConversion and GPUCheckResourceUsage. Callers
supply the leaf element bit width,
which lets the GPU check keep its index-bitwidth handling and the
recursion into shaped element types. On the CPU side the remaining
scalar steps are overflow-checked in place with `llvm::MulOverflow` /
`llvm::AddOverflow`. The pre-existing
`checkedAdd`/`checkedMul`/`checkedAlignTo` wrappers in
LLVMCPUAssignWorkgroupLocalMemory.cpp are replaced with
`llvm::MulOverflow` / `llvm::AddOverflow`.

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Signed-off-by: Zmicier Prybysh <zprybysh@baylibre.com>