The windows calling convention says that only XMM6-XMM15 are SOE(callee save), whereas upper 128 bits of YMM6-15 are SOC(caller save):
https://msdn.microsoft.com/en-us/library/9z1stfyw.aspx

The x86.ad file does not reflect this and shows the windows native calling convention as SOE for the entire YMM6-15.
The following excerpt from x86.ad file:
#ifdef _WIN64
reg_def XMM6 ( SOC, SOE, Op_RegF, 6, xmm6->as_VMReg());
reg_def XMM6b( SOC, SOE, Op_RegF, 6, xmm6->as_VMReg()->next(1));
reg_def XMM6c( SOC, SOE, Op_RegF, 6, xmm6->as_VMReg()->next(2));
reg_def XMM6d( SOC, SOE, Op_RegF, 6, xmm6->as_VMReg()->next(3));
reg_def XMM6e( SOC, SOE, Op_RegF, 6, xmm6->as_VMReg()->next(4));
reg_def XMM6f( SOC, SOE, Op_RegF, 6, xmm6->as_VMReg()->next(5));
reg_def XMM6g( SOC, SOE, Op_RegF, 6, xmm6->as_VMReg()->next(6));
reg_def XMM6h( SOC, SOE, Op_RegF, 6, xmm6->as_VMReg()->next(7));
...
#endif

If partial register spilling is not supported, one solution would be to declare the entire register as SOC.