Annotate imprecise FP division with fpaccuracy metadata

The OpenCL single precision division operation is only required to
be accurate to 2.5ulp.  Annotate the fdiv instruction with metadata
which signals to the backend that an imprecise divide instruction
may be used.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@143136 91177308-0d34-0410-b5e6-96231b3b80d8