sanitizer_common: allow COMMON_INTERCEPTOR_ENTER to use labels

The memcpy interceptor is the only one that uses COMMON_INTERCEPTOR_ENTER
more than once in a single function. This does not allow COMMON_INTERCEPTOR_ENTER
to use labels, because they are global for the whole function (not block scoped).
Don't include COMMON_INTERCEPTOR_ENTER code twice.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D105774