Use assert instead of branch-to-unreachable

(pretty sure we wrote this down in the style guide at some point)

Author: dwblaikie

llvm/lib/Target/X86/X86AsmPrinter.cpp

@@ -766,8 +766,8 @@ void X86AsmPrinter::emitStartOfAsmFile(Module &M) {
 
     if (FeatureFlagsAnd) {
       // Emit a .note.gnu.property section with the flags.
-      if (!TT.isArch32Bit() && !TT.isArch64Bit())
-        llvm_unreachable("CFProtection used on invalid architecture!");
+      assert((TT.isArch32Bit() || TT.isArch64Bit()) &&
+             "CFProtection used on invalid architecture!");
       MCSection *Cur = OutStreamer->getCurrentSectionOnly();
       MCSection *Nt = MMI->getContext().getELFSection(
           ".note.gnu.property", ELF::SHT_NOTE, ELF::SHF_ALLOC);