Fix a regression I introduced when I expanded the complex pseudos in
the Mips16 port. A few of the psuedos could either take signed
or unsigned arguments and I did not distinguish the case and improperly
rejected some valid cases that the assembler had previously accepted
when they were pure pseudos that expanded as assembly instructions.
Add test for ARM FastISel load/store register classes

r183624 fixed an issue that was tested indirectly. Test it directly with this new test.
Refactor: Move the IRAccess building code to a new function.