[CodeGen] Only consider innermost cast for !heapallocsite

Without opaque pointers, this code determined !heapallocsite based
on the innermost cast of the allocation call. With opaque pointers,
the casts no longer generate an instruction, so the outermost cast
is used. Add an explicit check for nested casts to prevent this.

Differential Revision: https://reviews.llvm.org/D145788