Enhance the sqlite3VdbeMemAboutToChange() shallow-copy validation mechanism

Enhance the sqlite3VdbeMemAboutToChange() shallow-copy validation mechanism
by adding the new OP_ReleaseReg opcode to tell MemAboutToChange() that a
range of registers is no longer needed so that the source register can be
freely changed.  This is a change to debugging and test builds only and
does not impact release builds.  Fix for ticket
[c62c5e58524b204d] and [5ad2aa6921faa1ee].  The previous fix to ticket
[5ad2aa6921faa1ee] is backed out by this change since this change is a better
fix.

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