Sema: support pointer subtraction

test/cases/compile_errors/invalid_pointer_arithmetic.zig

@@ -0,0 +1,52 @@
+export fn a(x: [*]u8) void {
+    _ = x * 1;
+}
+
+export fn b(x: *u8) void {
+    _ = x * x;
+}
+
+export fn c() void {
+    const x: []u8 = undefined;
+    const y: []u8 = undefined;
+    _ = x - y;
+}
+
+export fn d() void {
+    var x: [*]u8 = undefined;
+    var y: [*]u16 = undefined;
+    _ = &x;
+    _ = &y;
+    _ = x - y;
+}
+
+comptime {
+    const x: *u8 = @ptrFromInt(1);
+    const y: *u16 = @ptrFromInt(2);
+    _ = x - y;
+}
+
+comptime {
+    const x: [*]u0 = @ptrFromInt(1);
+    _ = x + 1;
+}
+
+comptime {
+    const x: *u0 = @ptrFromInt(1);
+    const y: *u0 = @ptrFromInt(2);
+    _ = x - y;
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :2:11: error: invalid pointer-integer arithmetic operator
+// :2:11: note: pointer-integer arithmetic only supports addition and subtraction
+// :6:11: error: invalid pointer-pointer arithmetic operator
+// :6:11: note: pointer-pointer arithmetic only supports subtraction
+// :12:11: error: invalid operands to binary expression: 'Pointer' and 'Pointer'
+// :20:11: error: incompatible pointer arithmetic operands '[*]u8' and '[*]u16'
+// :26:11: error: incompatible pointer arithmetic operands '*u8' and '*u16'
+// :31:11: error: pointer arithmetic requires element type 'u0' to have runtime bits
+// :37:11: error: pointer arithmetic requires element type 'u0' to have runtime bits