In -O2 mode, postLower() is supposed to iterate over just the
instructions that were most recently added.  Instead, it was iterating
all the way to the end of the block, also post-lowering high-level ICE
instructions that hadn't yet been lowered.  This was basically
harmless, given that the spec2k asm code is identical after this
patch, but it improves performance.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/721333004