fastalloc does not support arbitrary numbers of `any` defs allocating into spillslots

[cfallin]

After bytecodealliance/wasmtime#10502, a necessary prerequisite for exception handling in Cranelift (and thus Wasmtime), we generate all return-value defs for callsites as defs on the call instruction, with an any constraint. The default (backtracking) allocator is able to allocate defs past the number of registers into spillslots directly. However, it appears that fastalloc cannot do this: it aborts if a def with an any constraint cannot allocate a register.

See this fuzzbug for a reproduction.

[cfallin]

I'm filing this as a tracking issue and will disable fastalloc in Cranelift for now; it's a tier-3 feature and I don't have the cycles to update the allocator unfortunately. At some later time we may be able to fix this limitation and then re-enable it.