PNaCl: FlattenGlobals: Use ptrtoint+add instead of getelementptr+bitcast

The constructor for getelementptr ConstantExprs does some constant
folding which can add 1 or 2 more indexes to the getelementptr.  This
complicates checking for FlattenGlobals' normal form in the PNaCl ABI
checker.

Worse, the GCC torture tests turned up a pathological case where this
constant folding adds 4 indexes to the getelementptr and leaves the
original struct type behind:

    @q = global i8* getelementptr inbounds (
        %union.u* bitcast ([260 x i8]* @v to %union.u*),
        i32 0, i32 0, i32 0, i32 0, i32 4)

That comes from the following code in
gcc/testsuite/gcc.c-torture/execute/pr43784.c:

    struct s {
      unsigned char a[256];
    };
    union u {
      struct { struct s b; int c; } d;
      struct { int c; struct s b; } e;
    };
    static union u v;
    static struct s *q = &v.e.b;

We can fix this by using ptrtoint+add instead of
getelementptr+bitcast, because ConstantExpr won't automatically
convert ptrtoint to something else.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3113
TEST=*.ll tests + PNaCl toolchain trybots

Review URL: https://codereview.chromium.org/15647009

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