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It seems that most 3-address adds of temps can be move coalesced.
Therefore, we limit the behaviour added by 1567585dda8 to only affect
immediate adds.
Also, add conversion of immediate mov+sub to lea.
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branch and alub overlap in their use cases, but the backends rely on
knowing that the result is unused in their lowering of branch. By
extending alub so that the destination is optional, it can fully replace
branch.
This simplifies rtl by reducing code duplication and the number of
instructions.
Also, in the x86 and arm backends, we can now use 'test' and
{'tst','mvn','teq'} to lower some alubs without destinations. This is
particularly good for x86, as sequences such as 'is_boxed' type tests
now get shorter (both from not needing a mov to copy the variable, but
also from the fact that 'testb' encodes shorter than 'andq').
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This is primarily useful for heap allocations, as a two-address 'add'
can't be used to both copy the heap pointer to another register, and add
the tag.
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The x86 backend crashes if certain RTL optimisations were omitted,
preventing it from being usable at lower optimisation levels.
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RTL can produce an #fconv{} instruction with an immediate operand, but
the backends unconditionally access the operand as a temporary. This
results in broken representation in the backends and eventually they
crash.
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