evar_conv: Refine occur_rigidly
evar_conv: Refine occur_rigidly

This avoids postponing constraints which will surely produce
an occur-check and allow to backtrack on first-order unifications
producing those constraints directly (e.g. to apply eta).
(fixes HoTT/HoTT with 8.5).

SkySkimmer/coq,ejgallego/coq,silene/coq,coq/coq,ppedrot/coq,coq/coq,gares/coq,mattam82/coq,maximedenes/coq,SkySkimmer/coq,Zimmi48/coq,gares/coq,william-lawvere/coq,Zimmi48/coq,amintimany/coq,ejgallego/coq,letouzey/coq-wip,Matafou/coq,gares/coq,coq/coq,JasonGross/coq,Zimmi48/coq,SkySkimmer/coq,letouzey/coq-wip,letouzey/coq-wip,william-lawvere/coq,letouzey/coq-wip,Ptival/coq,gares/coq,letouzey/coq-wip,coq/coq,maximedenes/coq,ppedrot/coq,william-lawvere/coq,amintimany/coq,ejgallego/coq,Matafou/coq,Lionel-Rieg/coq,Matafou/coq,JasonGross/coq,Ptival/coq,JasonGross/coq,ppedrot/coq,mattam82/coq,silene/coq,Zimmi48/coq,Lionel-Rieg/coq,ppedrot/coq,maximedenes/coq,amintimany/coq,JasonGross/coq,Lionel-Rieg/coq,maximedenes/coq,mattam82/coq,SkySkimmer/coq,gares/coq,maximedenes/coq,Ptival/coq,silene/coq,ppedrot/coq,ejgallego/coq