Disable the optimization about promoting vector-element-access with symbolic index.

This optimization is unstable at this moment; it 
  1) block us on a very important application
  2) PR15200
  3) test6 and test7 in test/Transforms/ScalarRepl/dynamic-vector-gep.ll
     (the CHECK command compare the output against wrong result)

   I personally believe this optimization should not have any impact on the
autovectorized code, as auto-vectorizer is supposed to put gather/scatter
in a "right" way.  Although in theory downstream optimizaters might reveal 
some gather/scatter optimization opportunities, the chance is quite slim.

   For the hand-crafted vectorizing code, in term of redundancy elimination,
load-CSE, copy-propagation and DSE can collectively achieve the same result,
but in much simpler way. On the other hand, these optimizers are able to 
improve the code in a incremental way; in contrast, SROA is sort of all-or-none
approach. However, SROA might slighly win in stack size, as it tries to figure 
out a stretch of memory tightenly cover the area accessed by the dynamic index.

 rdar://13174884
 PR15200

llvm-svn: 178912