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

[Juan Manuel Martinez Caamaño via llvm-commits]

jmmartinez wrote:

Hello @nvjle ,
I've started cherry-picking one-by-one the commits refactoring `SSAIfConv` from https://github.com/llvm/llvm-project/pull/106415 .

Could you give it a look ?

I've also tagged `llvm::codegen` and all the backends using this pass:
- PowerPC
- SystemZ
- AMDGPU
- X86
- AArch64
- Hexagon

https://github.com/llvm/llvm-project/pull/107390