cmd/compile: optimize A->B->C Moves that include VarDefs
We have an existing optimization that recognizes
memory moves of the form A -> B -> C and converts
them into A -> C, in the hopes that the store to
B will be end up being dead and thus eliminated.
However, when A, B, and C are large types,
the front end sometimes emits VarDef ops for the moves.
This change adds an optimization to match that pattern.
This required changing an old compiler test.
The test assumed that a temporary was required
to deal with a large return value.
With this optimization in place, that temporary
ended up being eliminated.