1. Add load/store words from the stack

1. Add load/store words from the stack
2. As part of this, added assembly format FEXT_RI16_SP_explicit_ins and
moved other lines for FEXT_RI16 formats to be in the right place in the code.
3. Added mayLoad and mayStore assignements for the load/store instructions added and for ones already there that did not have this assignment.
4. Another patch will deal with the problem of load/store byte/halfword to the stack. This is a particular Mips16 problem.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@164811 91177308-0d34-0410-b5e6-96231b3b80d8