[mlir][sparse] moving inbound check for slice driven loop into before block of the...

[mlir][sparse] moving inbound check for slice driven loop into before block of the WhileOp

This patch changes the while loop generated for iterating over a fully reduced sparse level with affine index expression.
Before:
```
cont = true
while (cont) {
  if (inBound()) {
    ....
    cont = true;
  } else {
    cont = false;
  }
}
```
After:
```
while(inBound()) {
  ....
}
```

Reviewed By: K-Wu

Differential Revision: https://reviews.llvm.org/D152463