[ARM][ARMLoadStoreOptimizer]

- The load store optimizer is currently merging multiple loads/stores into VLDM/VSTM with more than 16 doubleword registers
- This is an UNPREDICTABLE instruction and shouldn't be done
- It looks like the Limit for how many registers included in a merge got dropped at some point so I am reintroducing it in this patch
- This fixes https://bugs.llvm.org/show_bug.cgi?id=38389

Differential Revision: https://reviews.llvm.org/D52085

llvm-svn: 342872