[llvm] Revert "[NFC][EarlyIfConverter] Turn SSAIfConv into a local variable (#107390)" (PR #111385)

[Juan Manuel Martinez Caamaño via llvm-commits]

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@@ -163,7 +163,8 @@ class SSAIfConv {
   void rewritePHIOperands();
 
 public:
-  SSAIfConv(MachineFunction &MF) {
+  /// runOnMachineFunction - Initialize per-function data structures.
+  void runOnMachineFunction(MachineFunction &MF) {
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jmmartinez wrote:

This `runOnMachineFunction` is confusing. I think I'll rename it to `init` later

https://github.com/llvm/llvm-project/pull/111385