Don't try to constant fold libm functions with non-finite arguments.

Don't try to constant fold libm functions with non-finite arguments.

Usually we wouldn't do this anyway because llvm_fenv_testexcept would return an
exception, but we have seen some cases where neither errno nor fenv detect an
exception on arm-linux.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@114893 91177308-0d34-0410-b5e6-96231b3b80d8