[InstCombine] Fix miscompilation in PR83947 (#83993)

https://github.com/llvm/llvm-project/blob/762f762504967efbe159db5c737154b989afc9bb/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp#L394-L407

Comment from @topperc:
> This transforms assumes the mask is a non-zero splat. We only know its
a splat and not provably all 0s. The mask is a constexpr that includes
the address of the global variable. We can't resolve the constant
expression to an exact value.

Fixes #83947.