Use pushq / popq instead of subq $8, %rsp / addq $8, %rsp to adjust stack in

Use pushq / popq instead of subq $8, %rsp / addq $8, %rsp to adjust stack in
prologue and epilogue if the adjustment is 8. Similarly, use pushl / popl if
the adjustment is 4 in 32-bit mode.

In the epilogue, takes care to pop to a caller-saved register that's not live
at the exit (either return or tailcall instruction).
rdar://8771137

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122783 91177308-0d34-0410-b5e6-96231b3b80d8