Auto merge of #23673 - nikomatsakis:issue-23319-binops-ng-5, r=pnkfelix

The current binary operator code assumed that if the LHS was a scalar (`i32` etc), then the RHS had to match. This is not true with multidispatch. This PR generalizes the existing code to (primarily) use the traits -- this also allows us to defer the precise type-checking when the types aren't fully known. The one caveat is the unstable SIMD types, which don't fit in with the current traits -- in that case, the LHS type must be known to be SIMD ahead of time.

There is one semi-hacky bit in that during writeback, for builtin operators, if the types resolve to scalars (i32 etc) then we clear the method override. This is because we know what the semantics are and it is more efficient to generate the code directly. It also ensures that we can use these overloaded operators in constants and so forth.

cc @japaric
cc @aturon 

Fixes #23319 (and others).

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