Change math intrinsic attributes from readonly to readnone. These
are operations that do not access memory but may be sensitive
to floating-point environment changes. LLVM does not attempt
to model FP environment changes, so this was unnecessarily conservative
and was getting on the way of some optimizations, in particular
SLP vectorization.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203037 91177308-0d34-0410-b5e6-96231b3b80d8