[AlwaysInliner] Emit inline remark only when successful

Failures in `InlineFunction()` are caught after D121722, but `emitInlinedIntoBasedOnCost()` should only be called when inlining is successful. This also removes an unnecessary call to `shouldInline()` which always returned `InlineCost::getAlways()`.

Reviewed By: kyulee, nikic

Differential Revision: https://reviews.llvm.org/D121946