Macros that expand into branches (e.g. assert!, trace!) tend to generate
multiple condition/consequent blocks that have the span of the whole macro
invocation, which is unhelpful. Keeping only the first such span seems to
give better mappings, so remove the others.
When a span corresponds to a macro invocation that is visible from the
function body, split it into two parts. The first part covers just the
macro name plus !, and the second part covers the rest of the macro
invocation. This seems to give better results for code that uses macros.