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[CallSiteSplitting] Do not perform callsite splitting inside landing pad

Description

[CallSiteSplitting] Do not perform callsite splitting inside landing pad

Summary:
If the callsite is inside landing pad, do not perform callsite splitting.

Callsite splitting uses utility function llvm::DuplicateInstructionsInSplitBetween, which eventually calls llvm::SplitEdge. llvm::SplitEdge calls llvm::SplitCriticalEdge with an assumption that the function returns nullptr only when the target edge is not a critical edge (and further assumes that if the return value was not nullptr, the predecessor of the original target edge always has a single successor because critical edge splitting was successful). However, this assumtion is not true because SplitCriticalEdge returns nullptr if the destination block is a landing pad. This invalid assumption results assertion failure.

Fundamental solution might be fixing llvm::SplitEdge to not to rely on the invalid assumption. However, it'll involve a lot of work because current API assumes that llvm::SplitEdge never fails. Instead, this patch makes callsite splitting to not to attempt splitting if the callsite is in a landing pad.

Attached test case will crash with assertion failure without the fix.

Reviewers: fhahn, junbuml, dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D45130