AMDGPU: Delete probably wrong constant folding of expm1

It's not really correct to implement this as exp(x) - 1, it was maybe
OK for the restricted float-as-double case handled here. There's not a
strong reason to special case it with the host function, as the
implementation naturally constant folds anyway. InstSimplify can fully
handle everything in it, so just running the inliner alone with a
constant argument produces the fully constant folded result. This also
had no tests.

https://reviews.llvm.org/D156892