The original code for 64-bit icmp lowering had separate cases for
eq/ne versus other conditions, mostly because eq/ne need two 32-bit
comparisons while the others need three.  However, with small changes,
we can handle everything uniformly, simplifying the code.

This gets thoroughly tested by the test_icmp cross test.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4162
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/1130023002