cmd/compile/internal/gc: handle array slice self-assign in esc.go
Instead of skipping all OSLICEARR, skip only ones with non-pointer
array type. For pointers to arrays, it's safe to apply the
self-assignment slicing optimizations.
Refactored the matching code into separate function for readability.
This is an extension to already existing optimization.
On its own, it does not improve any code under std, but
it opens some new optimization opportunities. One
of them is described in the referenced issue.