[llvm] [AArch64][GlobalISel] Protect against fdiv of 1 (PR #184063)

[Matt Arsenault via llvm-commits]

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@@ -6819,9 +6819,13 @@ bool CombinerHelper::matchRepeatedFPDivisor(
   if (!MI.getFlag(MachineInstr::MIFlag::FmArcp))
     return false;
 
+  auto IsOne = [this](Register X) {
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arsenm wrote:

This seems like a brittle check, can you more directly defend against the CSE?

https://github.com/llvm/llvm-project/pull/184063