optimize @intCast in llvm backend (#24739)

Author: IOKG04

src/codegen/llvm.zig

@@ -9234,8 +9234,7 @@ pub const FuncGen = struct {
 
         const dest_is_enum = dest_ty.zigTypeTag(zcu) == .@"enum";
 
-        safety: {
-            if (!safety) break :safety;
+        bounds_check: {
             const dest_scalar = dest_ty.scalarType(zcu);
             const operand_scalar = operand_ty.scalarType(zcu);
 
@@ -9254,7 +9253,7 @@ pub const FuncGen = struct {
                 };
             };
 
-            if (!have_min_check and !have_max_check) break :safety;
+            if (!have_min_check and !have_max_check) break :bounds_check;
 
             const operand_llvm_ty = try o.lowerType(pt, operand_ty);
             const operand_scalar_llvm_ty = try o.lowerType(pt, operand_scalar);
@@ -9272,12 +9271,16 @@ pub const FuncGen = struct {
                     const vec_ty = ok_maybe_vec.typeOfWip(&fg.wip);
                     break :ok try fg.wip.callIntrinsic(.normal, .none, .@"vector.reduce.and", &.{vec_ty}, &.{ok_maybe_vec}, "");
                 } else ok_maybe_vec;
-                const fail_block = try fg.wip.block(1, "IntMinFail");
-                const ok_block = try fg.wip.block(1, "IntMinOk");
-                _ = try fg.wip.brCond(ok, ok_block, fail_block, .none);
-                fg.wip.cursor = .{ .block = fail_block };
-                try fg.buildSimplePanic(panic_id);
-                fg.wip.cursor = .{ .block = ok_block };
+                if (safety) {
+                    const fail_block = try fg.wip.block(1, "IntMinFail");
+                    const ok_block = try fg.wip.block(1, "IntMinOk");
+                    _ = try fg.wip.brCond(ok, ok_block, fail_block, .none);
+                    fg.wip.cursor = .{ .block = fail_block };
+                    try fg.buildSimplePanic(panic_id);
+                    fg.wip.cursor = .{ .block = ok_block };
+                } else {
+                    _ = try fg.wip.callIntrinsic(.normal, .none, .assume, &.{}, &.{ok}, "");
+                }
             }
 
             if (have_max_check) {
@@ -9288,12 +9291,16 @@ pub const FuncGen = struct {
                     const vec_ty = ok_maybe_vec.typeOfWip(&fg.wip);
                     break :ok try fg.wip.callIntrinsic(.normal, .none, .@"vector.reduce.and", &.{vec_ty}, &.{ok_maybe_vec}, "");
                 } else ok_maybe_vec;
-                const fail_block = try fg.wip.block(1, "IntMaxFail");
-                const ok_block = try fg.wip.block(1, "IntMaxOk");
-                _ = try fg.wip.brCond(ok, ok_block, fail_block, .none);
-                fg.wip.cursor = .{ .block = fail_block };
-                try fg.buildSimplePanic(panic_id);
-                fg.wip.cursor = .{ .block = ok_block };
+                if (safety) {
+                    const fail_block = try fg.wip.block(1, "IntMaxFail");
+                    const ok_block = try fg.wip.block(1, "IntMaxOk");
+                    _ = try fg.wip.brCond(ok, ok_block, fail_block, .none);
+                    fg.wip.cursor = .{ .block = fail_block };
+                    try fg.buildSimplePanic(panic_id);
+                    fg.wip.cursor = .{ .block = ok_block };
+                } else {
+                    _ = try fg.wip.callIntrinsic(.normal, .none, .assume, &.{}, &.{ok}, "");
+                }
             }
         }