Handle call-clobbered ymm registers on Win64.

The Win64 calling convention has xmm6-15 as callee-saved while still
clobbering all ymm registers.

Add a YMM_HI_6_15 pseudo-register that aliases the clobbered part of the
ymm registers, and mark that as call-clobbered.  This allows live xmm
registers across calls.

This hack wouldn't be necessary with RegisterMask operands representing
the call clobbers, but they are not quite operational yet.

llvm-svn: 149088