[AMDGPU] Assembler: Swap operands of flat_store instructions to match AMD assembler

Historically, AMD internal sp3 assembler has flat_store* addr, data
format. To match existing code and to enable reuse, change LLVM
definitions to match.  Also update MC and CodeGen tests.

Differential Revision: http://reviews.llvm.org/D16927

Patch by: Nikolay Haustov

llvm-svn: 260694