Assert that PNaCl bitcode only uses 128-bit vector casts.

src/IceTargetLoweringX86BaseImpl.h

@@ -3216,6 +3216,8 @@ void TargetX86Base<TraitsType>::lowerCast(const InstCast *Instr) {
         // Bitcast requires equal type sizes, which isn't strictly the case
         // between scalars and vectors, but to emulate v4i8 vectors one has to
         // use v16i8 vectors.
+        assert(getFlags().getApplicationBinaryInterface() != ABI_PNaCl &&
+               "PNaCl only supports real 128-bit vectors");
         _movd(Dest, legalize(Src0, Legal_Reg | Legal_Mem));
       } else {
         _movp(Dest, legalizeToReg(Src0));