When a rule `uses REG ... yields %a`, the result %a is always a
temporary, never a regvar. If the EM code uses _stl_ to put the
result in a regvar, then ncg emits _mr_ to move %a to the regvar.
There are two ways to put the result in the regvar without %a:
1. Yield a token, as in `yields {MUL_RR, %2, %1}`, so that _stl_
can move the token to the regvar without using %a.
2. Provide a pattern, like `sli stl`, that just puts the result
in `{LOCAL, $2}` and not %a.
Allow some tokens, like SUM_RIS and XEQ, onto the stack; and add
tokens like MUL_RR, and patterns like `sli stl`.
Delete patterns for `stl lol` and `sdl ldl` to avoid an extra
temporary %a when the local is a regvar. Delete `lal sti lal loi`
because it would emit wrong code.
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