[SelectionDAG] Split very large token factors for chained stores to 64k chunks.

Similar to D55073. Without this change, the DAG combiner crashes on code
with more than 64k of stores in a single basic block that form parallelizable
chains.

No test case, as it would be very IR file.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D56740

llvm-svn: 351571