Fix a problem with mips16 stubs when calls are transformed during

Fix a problem with mips16 stubs when calls are transformed during
tail call optimization. Some more work may be needed for indirect
calls but this patch fixes the current regression in Prolangc++/trees.
S2 optimization as part of the general cleanup and optimization
of prolog and epilog was not saving S2 in this case and needed to.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197630 91177308-0d34-0410-b5e6-96231b3b80d8