Currently, STDSPQRr and STDWSPQRr are expanded only during
AVRFrameLowering - this means that if any of those instructions happen
to appear _outside_ of the typical FrameSetup / FrameDestroy
context, they wouldn't get substituted, eventually leading to a crash:
```
LLVM ERROR: Not supported instr: <MCInst XXX <MCOperand Reg:1>
<MCOperand Imm:15> <MCOperand Reg:53>>
```
This commit fixes this issue by moving expansion of those two opcodes
into AVRExpandPseudo.
This bug was originally discovered due to the Rust compiler_builtins
library. Its 0.1.37 release contained a 128-bit software
division/remainder routine that exercised this buggy branch in the code.
Reviewed By: benshi001
Differential Revision: https://reviews.llvm.org/D123528
