[mlir] MemRefToLLVM: Save / restore stack when lowering memref.copy

The MemRef to LLVM conversion pass emits `llvm.alloca` operations to promote MemRef descriptors to the stack when lowering `memref.copy` operations for operands which do not have a contiguous layout in memory. The original stack position is never restored after the allocations, which creates an issue when the copy operation is embedded into a loop with a high trip count, ultimately resulting in a segmentation fault due to the stack growing too large.

Below is as a minimal example illustrating the issue:

```
module {
  func.func @main() {
    %arg0 = memref.alloc() : memref<32x64xi64>
    %arg1 = memref.alloc() : memref<16x32xi64>
    %lb = arith.constant 0 : index
    %ub = arith.constant 100000 : index
    %step = arith.constant 1 : index
    %slice = memref.subview %arg0[16,32][16,32][1,1] :
       memref<32x64xi64> to memref<16x32xi64, #map>

    scf.for %i = %lb to %ub step %step {
       memref.copy %slice, %arg1 :
         memref<16x32xi64, #map> to memref<16x32xi64>
    }

    return
  }
}
```

When running the code above, e.g., with mlir-cpu-runner, the execution crashes with a segmentation fault:

```
$ mlir-opt \
    --convert-scf-to-cf \
    --convert-memref-to-llvm \
    --convert-func-to-llvm
    --convert-cf-to-llvm \
    --reconcile-unrealized-casts <file> | \
  mlir-cpu-runner \
    -e main -entry-point-result=void \
    --shared-libs=$PWD/build/lib/libmlir_c_runner_utils.so
[...]
Segmentation fault
```

This patch causes the code lowering a `memref.copy` operation in the MemRefToLLVM pass to emit a pair of matching `llvm.intr.stacksave` and `llvm.intr.stackrestore` operations around the promotion of memory descriptors and the subsequent call to `memrefCopy` in order to restore the stack to its original position after the call.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D135756