Fix codegen-issues with multi-edge phi #27609
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It is possible, after optimziations for the two branches of a conditional to go to the same basic block. We have an IR invariant that requires the incoming value to be the same for both incoming edges in this case. LLVM has the same invariant. However, translating from a julia-incoming-value to an LLVM one may require additional code generation, and without further GVN, LLVM does not know that the two values are identical and thus believes its invariants are violated. The fix is relatively straightforward. Since we don't have user-inserted switch terminators, we can simply emit a conditional branch with equal successors as unconditional branches and skip codegening one of the phi paths. This avoids the issue and also generates less code in this case. Fixes #27594
The problem here was that we had an incomplete phi from a multi-branch, but the logic that fills up the phi with `undef`s for incomplete phis, only added one entry to the phi node. Rather than the two that LLVM was expecting. Since the previous commit prevents us from turning conditional branches into multi-edge phis, this problem does not occur anymore. With this test, this fixes #27597
ararslan
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compiler:codegen
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See commit messages. Fixes #27594, #27597.