Implement negation of longs efficiently. For this testcase:

Implement negation of longs efficiently.  For this testcase:

long %test(long %X) {
        %Y = sub long 0, %X
        ret long %Y
}

We used to generate:

test:
        sub %ESP, 4
        mov DWORD PTR [%ESP], %ESI
        mov %ECX, DWORD PTR [%ESP + 8]
        mov %ESI, DWORD PTR [%ESP + 12]
        mov %EAX, 0
        mov %EDX, 0
        sub %EAX, %ECX
        sbb %EDX, %ESI
        mov %ESI, DWORD PTR [%ESP]
        add %ESP, 4
        ret

Now we generate:

test:
        mov %EAX, DWORD PTR [%ESP + 4]
        mov %EDX, DWORD PTR [%ESP + 8]
        neg %EAX
        adc %EDX, 0
        neg %EDX
        ret

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@12681 91177308-0d34-0410-b5e6-96231b3b80d8