Handle out-of-bound value for Gather alike operation

[chentong319]

Current situation: when a value of indices is out-of-bound, the execution is halted with assertion error.
This PR provides some improvements:

1. Do not generate assertion error. Instead, print out error message and replace with out-of-bound value with zero, with the hope that the execution will finish gracefully.
2. Print the out-of-bound value.
3. Share the code for GatherOp and GatherElementsOp. I did not define a common utility function because there are too many parameters. In future, we can merge all the lowering code, not just for enable-safe-code-gen, for the two ops.

[AlexandreEichenberger]

You could do a common function

generateDynBoundCheck0ToUb(rewriter, op, index, ub, message)

where you extract the message you want from the op, can append message, and generate the needed code.

[AlexandreEichenberger]

please add SCFBuilder to create object, and then you can use

  void ifThenElse(mlir::Value cond, SCFThenElseBodyFn thenFn,
      SCFThenElseBodyFn elseFn = nullptr) const;

to be in line with current practices.

[AlexandreEichenberger]

Why not capping at 0 on the lower side and axisDim on the upper size?

[chentong319]

No particular reason. Just feel 0 is a safer choice.

[AlexandreEichenberger]

Since index is already an IndexExpr, and that we have the dim of axis in the loop bounds, you could use index express all the way through as it has overloaded operators.

  IndexExpr operator==(IndexExpr const b) const;
  IndexExpr operator==(int64_t const b) const;
  IndexExpr operator!=(IndexExpr const b) const;
  IndexExpr operator!=(int64_t const b) const;
  IndexExpr operator<=(IndexExpr const b) const;
  IndexExpr operator<=(int64_t const b) const;
  IndexExpr operator<(IndexExpr const b) const;
  IndexExpr operator<(int64_t const b) const;
  IndexExpr operator>=(IndexExpr const b) const;
  IndexExpr operator>=(int64_t const b) const;
  IndexExpr operator>(IndexExpr const b) const;
  IndexExpr operator>(int64_t const b) const;

  IndexExpr operator&(IndexExpr const b) const;
  IndexExpr operator|(IndexExpr const b) const;
  IndexExpr operator!() const;