Draw STG F and D registers from the same pool of available SSE registers on x86-64.
On x86-64 F and D registers are both drawn from SSE registers, so there is no reason not to draw them from the same pool of available SSE registers. This means that whereas previously a function could only receive two Double arguments in registers even if it did not have any Float arguments, now it can receive up to 6 arguments that are any mix of Float and Double in registers. This patch breaks the LLVM back end. The next patch will fix this breakage.
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