Save a few spurious clang warnings in the generated code

[Antony Lee]

Compiling the following code

#cython: boundscheck=False

def main(double[:, ::1] x):

    return x[0]

with clang generates a bunch of warnings.  A few of them can be trivially avoided by marking a branch as knowingly dead:

testmod.c:1356:15: warning: code will never be executed [-Wunreachable-code]

    if (0 && (__pyx_tmp_idx < 0 || __pyx_tmp_idx >= __pyx_tmp_shape)) {

              ^~~~~~~~~~~~~

testmod.c:1356:9: note: silence by adding parentheses to mark code as explicitly dead

    if (0 && (__pyx_tmp_idx < 0 || __pyx_tmp_idx >= __pyx_tmp_shape)) {

        ^

        /* DISABLES CODE */ ( )

(i.e., write "if ((0) && ...)" instead of "if (0 &&)")

This heuristic was introduced in clang at this commit: http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20140324/102207.html

Antony