Now that we have real size changing ops, we don't need to marked high
bits of the destination as garbage. The goal of the optimizer is to
predict the value of the temps (and not of the registers) and do
simplifications when possible. The problem there is therefore not the
fact that those bits are not counted as garbage, but that a size
changing op is replaced by a move.
This patch is basically a revert of 24666baf, including the changes that
have been made since then.
Cc: Paolo Bonzini <address@hidden>
Cc: Richard Henderson <address@hidden>
Signed-off-by: Aurelien Jarno <address@hidden>