[mlir][scf] Canonicalize scf.for last tensor iteration result.

Canonicalize the iter_args of an scf::ForOp that involve a tensor_load and
for which only the last loop iteration is actually visible outside of the
loop. The canonicalization looks for a pattern such as:
```
   %t0 = ... : tensor_type
   %0 = scf.for ... iter_args(%bb0 : %t0) -> (tensor_type) {
     ...
     // %m is either tensor_to_memref(%bb00) or defined above the loop
     %m... : memref_type
     ... // uses of %m with potential inplace updates
     %new_tensor = tensor_load %m : memref_type
     ...
     scf.yield %new_tensor : tensor_type
   }
```

`%bb0` may have either 0 or 1 use. If it has 1 use it must be exactly a
`%m = tensor_to_memref %bb0` op that feeds into the yielded `tensor_load`
op.

If no aliasing write of `%new_tensor` occurs between tensor_load and yield
then the value %0 visible outside of the loop is the last `tensor_load`
produced in the loop.

For now, we approximate the absence of aliasing by only supporting the case
when the tensor_load is the operation immediately preceding the yield.

The canonicalization rewrites the pattern as:
```
   // %m is either a tensor_to_memref or defined above
   %m... : memref_type
   scf.for ... { // no iter_args
     ... // uses of %m with potential inplace updates
   }
   %0 = tensor_load %m : memref_type
```

Differential revision: https://reviews.llvm.org/D97953

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