Subzero: A few fixes toward running larger programs.

Subzero: A few fixes toward running larger programs.

1. Add 'llvm2ice -disable-globals' to disable Subzero translation of
global initializers, since full support isn't yet implemented.

2. Change the names of intra-block branch target labels to avoid
collisions with basic block labels.

3. Fix lowering of "br i1 <constant>, label ...", which was producing
invalid instructions like "cmp 1, 0".

4. Fix the "make format-diff" operation, which was diffing against the wrong target.

BUG= none
R=wala@chromium.org

Review URL: https://codereview.chromium.org/449093002