PPC32 cannot form counter loops around i64 FP conversions

On PPC32, i64 FP conversions are implemented using runtime calls (which clobber
the counter register). These must be excluded.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182023 91177308-0d34-0410-b5e6-96231b3b80d8