Improve vectorization diagnostic messages and extend vectorize(enable) pragma.

This patch changes the analysis diagnostics produced when loops with
floating-point recurrences or memory operations are identified. The new messages 
say "cannot prove it is safe to reorder * operations; allow reordering by
specifying #pragma clang loop vectorize(enable)". Depending on the type of 
diagnostic the message will include additional options such as ffast-math or
__restrict__.

This patch also allows the vectorize(enable) pragma to override the low pointer
memory check threshold. When the hint is given a higher threshold is used.

See the clang patch for the options produced for each diagnostic.

llvm-svn: 246187