64-bit Windows ymmh registers not properly preserved

In short, Windows defines xmm6-xmm15 registers to be callee-saved. Probably for back-compatibility reasons the ymmh6-15 parts (upper 128 bits) are volatile.

Spec: https://docs.microsoft.com/en-us/cpp/build/x64-software-conventions?view=vs-2017#register-usage

Currently DR does not preserve the ymmh6-15 states when context switching.

xref #1312