[lldb][nfc] early exit/continue

Summary:
This commit just tries to invert some `if`'s logic to
`return`/`continue` early.

Reviewers: jankratochvil, teemperor

Reviewed By: jankratochvil, teemperor

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D77377