[llvm] [InstCombine] Prevent infinite loop with two shifts (PR #118806)

[Nikita Popov via llvm-commits]

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nikic wrote:

The last two instructions in this test shouldn't be needed, it can be reduced to:
```
@c = external constant i8
@c2 = external constant i8

define i64 @testfunc() {
  %shl1 = shl i64 1, ptrtoint (ptr @c2 to i64)
  %shl2 = shl i64 %shl1, ptrtoint (ptr @c to i64)
  ret i64 %shl2
}
```.

https://github.com/llvm/llvm-project/pull/118806