[llvm] [NFC][InstCombine] Initialize potentially uninitialized local pointers (PR #208563)

[Younes Yazami via llvm-commits]

yelidris-public wrote:

@nikic "Files we don't modify" = files identical to upstream in our tree.
On overriding: /wd4703 technically suppresses C4703 even under /sdl, but our build's security-compliance policy requires warnings resolved in code, not suppressed

https://github.com/llvm/llvm-project/pull/208563