Fix invalid bitcast taking bool out of a union represented as a scalar

As reported in rust-lang#54668 (comment)

Author: ogoffart

src/librustc_codegen_ssa/mir/operand.rs

@@ -243,14 +243,22 @@ impl<'a, 'tcx: 'a, V: CodegenObject> OperandRef<'tcx, V> {
             _ => bug!("OperandRef::extract_field({:?}): not applicable", self)
         };
 
+        let bitcast = |bx: &mut Bx, val, ty| {
+            if ty == bx.cx().type_i1() {
+                bx.trunc(val, ty)
+            } else {
+                bx.bitcast(val, ty)
+            }
+        };
+
         // HACK(eddyb) have to bitcast pointers until LLVM removes pointee types.
         match val {
             OperandValue::Immediate(ref mut llval) => {
-                *llval = bx.bitcast(*llval, bx.cx().immediate_backend_type(field));
+                *llval = bitcast(bx, *llval, bx.cx().immediate_backend_type(field));
             }
             OperandValue::Pair(ref mut a, ref mut b) => {
-                *a = bx.bitcast(*a, bx.cx().scalar_pair_element_backend_type(field, 0, true));
-                *b = bx.bitcast(*b, bx.cx().scalar_pair_element_backend_type(field, 1, true));
+                *a = bitcast(bx, *a, bx.cx().scalar_pair_element_backend_type(field, 0, true));
+                *b = bitcast(bx, *b, bx.cx().scalar_pair_element_backend_type(field, 1, true));
             }
             OperandValue::Ref(..) => bug!()
         }

src/test/codegen/union-abi.rs

@@ -78,3 +78,9 @@ pub union CUnionU128{a:u128}
 #[no_mangle]
 pub fn test_CUnionU128(_: CUnionU128) { loop {} }
 
+pub union UnionBool { b:bool }
+// CHECK: define zeroext i1 @test_UnionBool(i8 %b)
+#[no_mangle]
+pub fn test_UnionBool(b: UnionBool) -> bool { unsafe { b.b }  }
+// CHECK: %0 = trunc i8 %b to i1
+