Do not always add -msse2 to release flags for icc

At least on Windows, this flag will override the -msse4.1 or -mavx
flags added to the main C flags. Since we already add -msse2 in
the same place when using SSE2, I see no reason why it should be
added to the release flags separately.

Change-Id: I933f5b1d5e4e4ead1635ec47b5272a0f6f69da67