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

llvm-svn: 122783