Subzero. Fixes a bug in the register allocator.

Subzero. Fixes a bug in the register allocator.

This bug was uncovered While implementing the llvm.nacl.atomic.cmpxchg
lowering for i64 for ARM32. For reference, the lowering is

retry:
    ldrexd     tmp_i, tmp_i+1 [addr]
    cmp        tmp_i+1, expected_i+1
    cmpeq      tmp_i, expected_i
    strexdeq   success, new_i, new_i+1, [addr]
    movne      expected_i+1, tmp_i+1
    movne      expected_i, tmp_i
    cmpeq      success, #0
    bne        retry
    mov        dest_i+1, tmp_i+1
    mov        dest_i, tmp_i

The register allocator would allocate r4 to both success and new_i,
which is clearly wrong (expected_i is alive thought the cmpxchg loop.)
Adding a fake-use(new_i) after the loop caused the register allocator
to fail due to the impossibility to allocate a register for an infinite
weight register. The problem was being caused for not evicting live
ranges that were assigned registers that alias the selected register.

BUG=
R=kschimpf@google.com, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1373823006 .