Change StackDirection from StackGrowsUp to StackGrowsDown.

The following improvements are accomplished as a result of applying this patch:
- Fixed frame objects' offsets (relative to either the virtual frame pointer or
  the stack pointer) are set before instruction selection is completed. There is
  no need to wait until Prologue/Epilogue Insertion is run to set them.
- Calculation of final offsets of fixed frame objects is straightforward. It is
  no longer necessary to assign negative offsets to fixed objects for incoming
  arguments in order to distinguish them from the others.
- Since a fixed object has its relative offset set during instruction
  selection, there is no need to conservatively set its alignment to 4.
- It is no longer necessary to reorder non-fixed frame objects in 
  MipsFrameLowering::adjustMipsStackFrame.

llvm-svn: 131915