[DAGCombiner] Exploiting more about the transformation of TransformFPLoadStorePair function

For a given floating point load / store pair, if the load value isn't used by any other operations, 
then consider transforming the pair to integer load / store operations if the target deems the transformation profitable.

And we can exploiting much more when there are other operation nodes with chain operand between the load/store pair 
so long as we keep the chain ordering original. We only replace the register used to load/store from float to integer.

I only add testcase in ARM because the TLI.isDesirableToTransformToIntegerOp hook is only enabled in ARM target.

Differential Revision: https://reviews.llvm.org/D60601

llvm-svn: 364883

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