ISLE standard prelude: Additional types and helpers

In preparing to move the s390x back-end to ISLE, I noticed a few
missing pieces in the common prelude code.  This patch:

- Defines the reference types $R32 / $R64.
- Provides a trap_code_bad_conversion_to_integer helper.
- Provides an avoid_div_traps helper.  This requires passing the
  generic flags in addition to the ISA-specifc flags into the
  ISLE lowering context.

cranelift/codegen/src/isa/x64/lower.rs

@@ -1190,7 +1190,7 @@ fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
         None
     };
 
-    if let Ok(()) = isle::lower(ctx, isa_flags, &outputs, insn) {
+    if let Ok(()) = isle::lower(ctx, flags, isa_flags, &outputs, insn) {
         return Ok(());
     }