Fix iN/uN type literal width check for multiples of 8 (#6949)

The diagnostic requires bit widths to be multiples of 8, but the test
used a mask of 3 (lower two bits), which only enforces multiples of 4.
Use a mask of 7 so values like 12 incorrectly pass the check.

toolchain/check/handle_literal.cpp

@@ -90,7 +90,7 @@ static auto HandleIntOrUnsignedIntTypeLiteral(Context& context,
                                               Parse::NodeId node_id,
                                               SemIR::IntKind int_kind,
                                               IntId size_id) -> bool {
-  if (!(context.ints().Get(size_id) & 3).isZero()) {
+  if (!(context.ints().Get(size_id) & 7).isZero()) {
     CARBON_DIAGNOSTIC(IntWidthNotMultipleOf8, Error,
                       "bit width of integer type literal must be a multiple of "
                       "8; use `Core.{0:Int|UInt}({1})` instead",

toolchain/check/testdata/primitives/type_literals.carbon

@@ -82,6 +82,16 @@ var test_i1: i1;
 // CHECK:STDERR:               ^~~
 // CHECK:STDERR:
 var test_i15: i15;
+// CHECK:STDERR: fail_iN_bad_width.carbon:[[@LINE+4]]:14: error: bit width of integer type literal must be a multiple of 8; use `Core.Int(4)` instead [IntWidthNotMultipleOf8]
+// CHECK:STDERR: var test_i4: i4;
+// CHECK:STDERR:              ^~
+// CHECK:STDERR:
+var test_i4: i4;
+// CHECK:STDERR: fail_iN_bad_width.carbon:[[@LINE+4]]:15: error: bit width of integer type literal must be a multiple of 8; use `Core.Int(12)` instead [IntWidthNotMultipleOf8]
+// CHECK:STDERR: var test_i12: i12;
+// CHECK:STDERR:               ^~~
+// CHECK:STDERR:
+var test_i12: i12;
 // CHECK:STDERR: fail_iN_bad_width.carbon:[[@LINE+7]]:23: error: binding pattern has incomplete type `i1000000000` in name binding declaration [IncompleteTypeInBindingDecl]
 // CHECK:STDERR: var test_i1000000000: i1000000000;
 // CHECK:STDERR:                       ^~~~~~~~~~~
@@ -119,6 +129,16 @@ var test_u1: u1;
 // CHECK:STDERR:               ^~~
 // CHECK:STDERR:
 var test_u15: u15;
+// CHECK:STDERR: fail_uN_bad_width.carbon:[[@LINE+4]]:14: error: bit width of integer type literal must be a multiple of 8; use `Core.UInt(4)` instead [IntWidthNotMultipleOf8]
+// CHECK:STDERR: var test_u4: u4;
+// CHECK:STDERR:              ^~
+// CHECK:STDERR:
+var test_u4: u4;
+// CHECK:STDERR: fail_uN_bad_width.carbon:[[@LINE+4]]:15: error: bit width of integer type literal must be a multiple of 8; use `Core.UInt(12)` instead [IntWidthNotMultipleOf8]
+// CHECK:STDERR: var test_u12: u12;
+// CHECK:STDERR:               ^~~
+// CHECK:STDERR:
+var test_u12: u12;
 // CHECK:STDERR: fail_uN_bad_width.carbon:[[@LINE+7]]:23: error: binding pattern has incomplete type `u1000000000` in name binding declaration [IncompleteTypeInBindingDecl]
 // CHECK:STDERR: var test_u1000000000: u1000000000;
 // CHECK:STDERR:                       ^~~~~~~~~~~