[llvm] Reland [SimplifyCFG] Delete the unnecessary range check for small mask operation (PR #70542)

[Nikita Popov via llvm-commits]

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@@ -6631,6 +6630,27 @@ static bool SwitchToLookupTable(SwitchInst *SI, IRBuilder<> &Builder,
 
   BranchInst *RangeCheckBranch = nullptr;
 
+  // Grow the table to cover all possible index values to avoid the range check.
+  // It will use the default result to fill in the table hole later, so make
+  // sure it exist.
+  if (UseSwitchConditionAsTableIndex && HasDefaultResults) {
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nikic wrote:

To clarify: If the default is already not reachable, then there is no point in increasing the table size in the first place, as no branch will be generated.

https://github.com/llvm/llvm-project/pull/70542