FWIW, MIPS64 does not have this problem, as it uses useInt64OrConstant(AtStart) here, we're running into this on ARM64 only because we (possibly I) were too lazy to implement support for multiply-by-constant earlier.
Bug 1707774 Comment 5 Edit History
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MIPS64 does not have this problem for multiply, as it uses useInt64OrConstant(AtStart) here, we're running into this on ARM64 only because we (possibly I) were too lazy to implement support for multiply-by-constant earlier. MWasmStore could be similarly afflicted on ARM64 and MIPS64 because it has a useInt64RegisterAtStart for an rhs operand that could plausibly be a constant. There appear to be no other uses of useInt64RegisterAtStart in non-lhs contexts.