[mlir] do not use llvm.cmpxchg with floats

According to the LLVM Language Reference, 'cmpxchg' accepts integer or pointer
types. Several MLIR tests were using it with floats as it appears possible to
programmatically construct and print such an instruction, but it cannot be
parsed back. Use integers instead.

Depends On D85899

Reviewed By: flaub, rriddle

Differential Revision: https://reviews.llvm.org/D85900