[mlir][Linalg] Clarify error message in YieldOp verification NFC

The number of values yielded from a LinalgOp's payload has to match
the number of inits / outs operands of the LinalgOp.
These two numbers got mixed up in the respective error message, this
patch clarifies the message and updates the tests.

Reviewed By: nicolasvasilache, mehdi_amini

Differential Revision: https://reviews.llvm.org/D153124