Make comparisons against the null pointer as efficient as integer comparisons

Make comparisons against the null pointer as efficient as integer comparisons
against zero.  In particular, don't emit:

        mov %ESI, 0
        cmp %ECX, %ESI

instead, emit:

       test %ECX, %ECX

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@13407 91177308-0d34-0410-b5e6-96231b3b80d8