Simplify asynccheckEvaluator on Z

[fjeremic]

@joransiu suggested a small code improvement in #742 (comment) to the asynccheckEvaluator which can simplify one of the paths:

https://github.com/eclipse/openj9/blob/94649e13230cf293f73f060f83b331eb38174d8b/runtime/compiler/z/codegen/J9TreeEvaluator.cpp#L2189-L2211

As per the review comment, the code in lowerAsyncCheck will have ensured the memref we compare against will not have a base and an index register. If in case this does happen for whatever reason, the code in generateSILInstruction has us taken care of because it will separate the index register:

https://github.com/eclipse/omr/blob/840473a5de22834948bed9a8acc5bfb36a6ad10b/compiler/z/codegen/S390GenerateInstructions.cpp#L1565-L1568