Rollup merge of #155639 - oli-obk:builtin-binop-const, r=nnethercote

BinOpAssign always returns unit

I don't know why we treated assign ops as returning their binop type sometimes, but it's usually ignored later anyway and mostly affects infer vars.

Also updated a comment from 11 years ago when SIMD types apparently had builtin `==` logic.

Author: JonathanBrouwer