Fix assignments inside inline function expressions

[Rich-Harris]

The instrumentation was wonky for two reasons:

• it was inserting code before semi-colons that should have gone after semi-colons
• it was tracking which assignments needed instrumentation on the way 'down' the AST, meaning they were getting applied prematurely if a suitable place (i.e. the next expression statement it found) was found further in. now, it tracks those assignments on the way 'up'

fixes #3038