[SLP] Change extension of the test, NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304829 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/test/Transforms/SLPVectorizer/X86/reverse_extract_elements.cpp b/test/Transforms/SLPVectorizer/X86/reverse_extract_elements.ll
similarity index 100%
rename from test/Transforms/SLPVectorizer/X86/reverse_extract_elements.cpp
rename to test/Transforms/SLPVectorizer/X86/reverse_extract_elements.ll