Defines new PDLInterp operations needed for multi-root matching in PDL.

Defines new PDLInterp operations needed for multi-root matching in PDL.

This is commit 1 of 4 for the multi-root matching in PDL, discussed in https://llvm.discourse.group/t/rfc-multi-root-pdl-patterns-for-kernel-matching/4148 (topic flagged for review).

These operations are:
* pdl.get_accepting_ops: Returns a list of operations accepting the given value or a range of values at the specified position. Thus if there are two operations `%op1 = "foo"(%val)` and `%op2 = "bar"(%val)` accepting a value at position 0, `%ops = pdl_interp.get_accepting_ops of %val : !pdl.value at 0` will return both of them. This allows us to traverse upwards from a value to operations accepting the value.
* pdl.choose_op: Iteratively chooses one operation from a range of operations. Therefore, writing `%op = pdl_interp.choose_op from %ops` in the example above will select either `%op1`or `%op2`.

Testing: Added the corresponding test cases to mlir/test/Dialect/PDLInterp/ops.mlir.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D108543