[clang] Skip auto-init on scalar vars that have a non-constant Init and no self-ref (#94642)

In that scalar case, the Init should initialize the auto var before use. The Init might use uninitialized memory from other sources (e.g., heap) but auto-init did not help us in that case because the auto-init would have been overwritten by the Init before use.

For non-scalars e.g., classes, the Init expr might be a ctor call that leaves uninitialized members, so we leave the auto-init there.

The motivation is to have less IR for the optimizer to later remove, which may not be until a fairly late pass (DSE) or may not get optimized in lower optimization levels like O1 (no DSE) or sometimes due to derefinement.
This is ~10% less left-over auto-init in O1 in a few examples checked.

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