[VecLib] Update test case to match new vectorizer behaviour

The IR in clang/test/CodeGen/libpgmath-logfun-aarch64.ll can now be
vectorized by a factor of two. The IR still does not call the unsupported
libpgmath functions, as per the intention of the test.

Author: bryanpkc

clang/test/CodeGen/libpgmath-logfun-aarch64.ll

@@ -45,10 +45,8 @@ L.entry:
   ret void
 
 ; CHECK-NOT: __pd_log_4
-; CHECK: __pd_log_1
-; CHECK: __pd_log_1
-; CHECK: __pd_log_1
-; CHECK: __pd_log_1
+; CHECK: __pd_log_2
+; CHECK: __pd_log_2
 }
 
 ; Function Attrs: nounwind readnone willreturn