Add the br_icmp instruction.

This instruction behaves like icmp fused with brnz, and it can be used
to represent fused compare+branch instruction on Intel when optimizing
for macro-op fusion.

RISC-V provides compare-and-branch instructions directly, and it is
needed there too.

lib/cretonne/src/ir/instructions.rs

@@ -200,6 +200,13 @@ pub enum InstructionData {
         destination: Ebb,
         args: ValueList,
     },
+    BranchIcmp {
+        opcode: Opcode,
+        ty: Type,
+        cond: IntCC,
+        destination: Ebb,
+        args: ValueList,
+    },
     BranchTable {
         opcode: Opcode,
         ty: Type,
@@ -303,6 +310,9 @@ impl InstructionData {
             &InstructionData::Branch { destination, ref args, .. } => {
                 BranchInfo::SingleDest(destination, &args.as_slice(pool)[1..])
             }
+            &InstructionData::BranchIcmp { destination, ref args, .. } => {
+                BranchInfo::SingleDest(destination, &args.as_slice(pool)[2..])
+            }
             &InstructionData::BranchTable { table, .. } => BranchInfo::Table(table),
             _ => BranchInfo::NotABranch,
         }

lib/cretonne/src/verifier.rs

@@ -218,11 +218,9 @@ impl<'a> Verifier<'a> {
             &MultiAry { ref args, .. } => {
                 self.verify_value_list(inst, args)?;
             }
-            &Jump { destination, ref args, .. } => {
-                self.verify_ebb(inst, destination)?;
-                self.verify_value_list(inst, args)?;
-            }
-            &Branch { destination, ref args, .. } => {
+            &Jump { destination, ref args, .. } |
+            &Branch { destination, ref args, .. } |
+            &BranchIcmp { destination, ref args, .. } => {
                 self.verify_ebb(inst, destination)?;
                 self.verify_value_list(inst, args)?;
             }

lib/cretonne/src/write.rs

@@ -274,15 +274,19 @@ pub fn write_operands(w: &mut Write, dfg: &DataFlowGraph, inst: Inst) -> Result
         }
         Branch { destination, ref args, .. } => {
             let args = args.as_slice(pool);
-            if args.len() == 1 {
-                write!(w, " {}, {}", args[0], destination)
-            } else {
-                write!(w,
-                       " {}, {}({})",
-                       args[0],
-                       destination,
-                       DisplayValues(&args[1..]))
+            write!(w, " {}, {}", args[0], destination)?;
+            if args.len() > 1 {
+                write!(w, "({})", DisplayValues(&args[1..]))?;
             }
+            Ok(())
+        }
+        BranchIcmp { cond, destination, ref args, .. } => {
+            let args = args.as_slice(pool);
+            write!(w, " {}, {}, {}, {}", cond, args[0], args[1], destination)?;
+            if args.len() > 2 {
+                write!(w, "({})", DisplayValues(&args[2..]))?;
+            }
+            Ok(())
         }
         BranchTable { arg, table, .. } => write!(w, " {}, {}", arg, table),
         Call { func_ref, ref args, .. } => {