Only attempt to detect AVG if SSE2 is available

Summary:
In PR29973 Sanjay Patel reported an assertion failure when a certain
loop was optimized, for a target without SSE2 support.  It turned out
this was because of the AVG pattern detection introduced in rL253952.

Prevent the assertion failure by bailing out early in
`detectAVGPattern()`, if the target does not support SSE2.

Also add a minimized test case.

Reviewers: congh, eli.friedman, spatel

Subscribers: emaste, llvm-commits

Differential Revision: http://reviews.llvm.org/D20905