Unify RIR SSA transform into single pass, fix mapping bugs

[Stefan J. Wernli (swernli)]

This is fixes #3908 with two related RIR pass bugs that needed separate tweaks:

1. The SSA transform was performed internally via two passes across the blocks (two separate calls to the map_variable_use_in_block utility), but some information was lost after the first pass removed all store instructions. Unifying to a single pass with necessary phi node insertion computed from predecessors ahead of time prevents the loss of context.
2. For the Adaptive-only pruning of unneeded stores, old variable mappings were persisted even after a store may change what value a variable corresponds to, so removing the mapping when a store is NOT pruned prevents out of date constant values from propagating within a block.

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