Fix sroa replace constant with non constant in entry block #6591
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ReplaceConstantWithInst(C, V) replaces uses of C in the current function with V. If such a use C is an instruction I, the it replaces uses of C in I with V. However, this function did not make sure to only perform this replacement if V dominates I. As a result, it may end up replacing uses of C in instructions before the definition of V. The fix is to lazily compute the dominator tree in ReplaceConstantWithInst so that we can guard the replacement with that dominance check.
In order to replace memcpy instances, sometimes constants need to be replaced with instructions. Rather than place them wherever is immediately convenient, placing them in the entry block after the allocas ensures they have access to the allocas while making sure they dominate any uses, increasing the chances that the associated memcpy can be replaced rather than potentially introducing a domination issue that would prevent the memcpy replace.
We try to keep the allocas at the top of the entry block. This initialization code placed the init instruction just after the alloca it was initializing, which left it in the middle of the allocas. This disrupted future attempts to find the end of the allocas and potentially put instructions referencing allocas before the relevant alloca.
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A variation on #6556 that I'm not sure generates the right code just yet. The first commit slurps up the tests from #6556. The HLSL test passes without crashing, this change preserves the memcpy replacement so the .ll test doesn't pass. The concern is with the resulting copy seeming to be pointless.