When emitting the function epilog, check to see if there already a stack

When emitting the function epilog, check to see if there already a stack
adjustment.  If so, we merge the adjustment into the existing one.  This
allows us to generate:

caller2:
        sub %ESP, 12
        mov DWORD PTR [%ESP], 0
        mov %EAX, 1234567890
        mov %EDX, 0
        call func2
        add %ESP, 8
        ret 4

intead of:

caller2:
        sub %ESP, 12
        mov DWORD PTR [%ESP], 0
        mov %EAX, 1234567890
        mov %EDX, 0
        call func2
        sub %ESP, 4
        add %ESP, 12
        ret 4

for X86/fast-cc-merge-stack-adj.ll

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@22038 91177308-0d34-0410-b5e6-96231b3b80d8