[AlwaysInliner] Check inliner errors even without assserts

When we build clang without asserts we should still check the result of
`InlineFunction()` to be sure there wasn't an error. Otherwise we could
incorrectly merge attributes in the next line.

This also removes a redundent call to `getCaller()`.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D121722