Improve code generation of long shifts by 32.

Improve code generation of long shifts by 32.
On this testcase:

long %test(long %X) {
        %Y = shr long %X, ubyte 32
        ret long %Y
}

instead of:
t:
        mov %EAX, DWORD PTR [%ESP + 4]
        mov %EAX, DWORD PTR [%ESP + 8]
        sar %EAX, 0
        mov %EDX, 0
        ret

we now emit:
test:
        mov %EAX, DWORD PTR [%ESP + 4]
        mov %EAX, DWORD PTR [%ESP + 8]
        mov %EDX, 0
        ret

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@12688 91177308-0d34-0410-b5e6-96231b3b80d8