Fix a bug that caused multiple values to appear where they shouldn't.
Fix a bug that caused multiple values to appear where they shouldn't.

(LET ((X (VALUES 1 2))) X) and similar forms were being optimized to
(VALUES 1 2). They're now correctly converted to (VALUES (VALUES 1 2)),
limiting to a single value.

rvedam/Mezzano,froggey/Mezzano,varjagg/Mezzano,froggey/Mezzano,AugustusHuang/Mezzano,AugustusHuang/Mezzano,rvedam/Mezzano,varjagg/Mezzano