Convert another batch of straightforward handlers to named DSL
temporaries: Increment, Decrement, Not, GetLexicalEnvironment, Return,
and End. The migration uncovers a pleasing property of the new
pipeline: when a migrated handler invokes an un-migrated macro
(pop_inline_frame_and_resume here, which still uses positional t2/t3
/t4 internally), the macro body's positional uses appear as physical-
register defs in the flat instruction list and the allocator's
liveness analysis avoids placing live named temps in those registers.
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