checker: fix infix arithmetic on explicit struct pointers

[wenxuanjun]

Allow infix pointer arithmetic on explicit struct pointers and pointers to struct pointers.

Previously, pointer arithmetic only worked with primitive pointers like &int or &&int, but now it also works with struct pointers like &Struct and &&Struct.

This change keeps the fix local to infix pointer arithmetic:

• explicit struct pointers now work with + and -
• safe code still reports the existing unsafe warning
• invalid pointer operators like % continue to fail, but without the extra mismatched-type noise