Improve code for passing constant longs as arguments to function calls.

Improve code for passing constant longs as arguments to function calls.
For example, on this instruction:

        call void %test(long 1234)

Instead of this:
        mov %EAX, 1234
        mov %ECX, 0
        mov DWORD PTR [%ESP], %EAX
        mov DWORD PTR [%ESP + 4], %ECX
        call test

We now emit this:
        mov DWORD PTR [%ESP], 1234
        mov DWORD PTR [%ESP + 4], 0
        call test

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@12686 91177308-0d34-0410-b5e6-96231b3b80d8