[Arm][AsmParser] Restrict register list size for VSTM/VLDM

- The assembler accepts VSTM/VLDM with register lists (specifically double registers lists) with more than 16 registers specified
- The Arm architecture reference manual says this instruction must not contain more than 16 registers when the registers are doubleword registers
- This addresses one of the concerns in https://bugs.llvm.org/show_bug.cgi?id=38389

Differential Revision: https://reviews.llvm.org/D52082

llvm-svn: 342891