Add test case for C++ exception handling and fix the following mistakes in MipsFrameL...

Add test case for C++ exception handling and fix the following mistakes in MipsFrameLowering::emitPrologue:

- cfi directives are not inserted at the right location or in the right order.
- The source MachineLocation for the cfi directive that changes the cfa register
  to $fp should be MachineLocation::VirtualFP.
- A PROLOG_LABEL that marks the beginning of cfi_offset directives for
  callee-saved register is emitted even when no callee-saved registers are
  saved.
- When a callee-saved double precision register is saved, two cfi_offset
  directives, one for each of the paired single precision registers, should be
  emitted.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132703 91177308-0d34-0410-b5e6-96231b3b80d8