Add the saving of S2. This is needed for some of the floating point

Add the saving of S2. This is needed for some of the floating point
helper functions. This can be optimized out later when the remaining
parts of the helper function work is moved into the Mips16HardFloat pass.
For now it forces us to use the 32 bit save/restore instructions instead
of the 16 bit ones.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187712 91177308-0d34-0410-b5e6-96231b3b80d8