replacements and getHostProviders

[Deneau, Tom]

Doug or others --

Currently the HSAIL backend lazily delegates all replacements to the host.
And in HSAILHotSpotBackendFactory we also define our backend registers to be
        HotSpotRegisters registers = new HotSpotRegisters(Register.None, Register.None, Register.None);

expecting that to hit an assert if anything tries to get the threadRegister, heapBaseRegister or stackPointerRegister.

But if the method we are compiling in the HSAIL backend starts expanding a snippet that might use the threadRegister,
I see that HotSpotReplacementsUtil defines threadRegister() as

    public static Register threadRegister() {
        return runtime().getHostProviders().getRegisters().getThreadRegister();
    }

which always goes to the host rather than the backend's registers.  So we don't catch an error at compile time but
instead at runtime.

Is it a mistake that HotSpotReplacementsUtil goes thru the HostProviders?

I realize that in a real solution we should not delegate all replacements to the host, just the ones we know we
can implement, but just wondering about the above question.

-- Tom