Use -ftree-vectorize for autovec (#4971)

Summary:
X-link: https://github.com/facebookresearch/FBGEMM/pull/1989

I recall the flag was used before..

Pull Request resolved: #4971

Reviewed By: cthi

Differential Revision: D83827111

Pulled By: q10

fbshipit-source-id: 19affbb8343babaf6b0170c63a2a450dc58d7fd4

Author: cyyever

CMakeLists.txt

@@ -405,6 +405,8 @@ if(NOT DISABLE_FBGEMM_AUTOVEC)
       OBJECT
     SRCS
       ${FBGEMM_AUTOVEC_SRCS}
+    CC_FLAGS
+      -ftree-vectorize
     ENABLE_IPO
       ${FBGEMM_USE_IPO}
     DEPS