[memcpyopt] Restructure store(load src, dest) form of callslotopt for compile time

The search for the clobbering call is fairly expensive if uses are not optimized at construction.  Defer the clobber walk to the point in the implementation we need it; there are a bunch of bailouts before that point.  (e.g. If the source pointer is not an alloca, we can't do callslotopt.)

On a test case which involves a bunch of copies from argument pointers, this switches memcpyopt from > 1/2 second to < 10ms.