Return bools in GPR registers.

Boolean types are returned in %rax, so regclass_for_abi_type() should
return GPR.

Fixes #179.

cranelift/filetests/isa/intel/abi-bool.cton

@@ -0,0 +1,20 @@
+test compile
+set is_64bit=1
+isa intel haswell
+
+function %foo(i64, i64, i64, i32) -> b1 native {
+ebb3(v0: i64, v1: i64, v2: i64, v3: i32):
+    v5 = icmp ne v2, v2
+    v8 = iconst.i64 0
+    jump ebb2(v8, v3, v5)
+
+ebb2(v10: i64, v30: i32, v37: b1):
+    v18 = load.i32 notrap aligned v2
+    v27 = iadd.i64 v10, v10
+    v31 = icmp eq v30, v30
+    brz v31, ebb2(v27, v30, v37)
+    jump ebb0(v37)
+
+ebb0(v35: b1):
+    return v35
+}

lib/cretonne/src/isa/intel/abi.rs

@@ -125,7 +125,11 @@ pub fn legalize_signature(sig: &mut ir::Signature, flags: &shared_settings::Flag
 
 /// Get register class for a type appearing in a legalized signature.
 pub fn regclass_for_abi_type(ty: ir::Type) -> RegClass {
-    if ty.is_int() { GPR } else { FPR }
+    if ty.is_int() || ty.is_bool() {
+        GPR
+    } else {
+        FPR
+    }
 }
 
 /// Get the set of allocatable registers for `func`.