Check that APFloat::convert is not trying to target

ppc long double, which doesn't work.

This may break some stuff temporarily, but I want to
avoid the propagation of code that assumes this works.

llvm-svn: 49983

llvm/lib/Support/APFloat.cpp

@@ -1678,6 +1678,7 @@ APFloat::convert(const fltSemantics &toSemantics,
   opStatus fs;
 
   assertArithmeticOK(*semantics);
+  assertArithmeticOK(toSemantics);
   lostFraction = lfExactlyZero;
   newPartCount = partCountForBits(toSemantics.precision + 1);
   oldPartCount = partCount();