Modify MipsFrameLowering::emitPrologue and emitEpilogue.

- Use MipsAnalyzeImmediate to expand immediates that do not fit in 16-bit.
- Change the types of variables so that they are sufficiently large to handle
  64-bit pointers.
- Emit instructions to set register $28 in a function prologue after
  instructions which store callee-saved registers have been emitted. 
 

llvm-svn: 148917