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
This should fix assertion failures ("Requires at least SSE2!") when
building the games/0ad port with CPUTYPE=pentium3.
Reported by: madpilot
b5e99283f4