Free ReturnOp from being restricted to a FuncOp

[bondhugula]

• allow return op to be used from the top level of any op with a region (as opposed to just from the top level of a func op)
• the return operand count/types should match the op results' (whenever it's not in a FuncOp's region).

This allows std.return to be used both in a declarative (FuncOp) as well as an imperative (typical op) setting --- avoiding the need to create custom return ops for what is standard, in cases where control is to be transferred to the enclosing operation, and return values need to be modeled in ops with regions.

This also enables affine.graybox to hold a list of blocks with terminators.

Signed-off-by: Uday Bondhugula uday@polymagelabs.com

[bondhugula]

This was moved to MLIR in LLVM project.