Fix regalloc assignment of overlapping registers
SplitEditor::defFromParent() can create a register copy.
If register is a tuple of other registers and not all lanes are used
a copy will be done on a full tuple regardless. Later register unit
for an unused lane will be considered free and another overlapping
register tuple can be assigned to a different value even though first
register is live at that point. That is because interference only look at
liveness info, while full register copy clobbers all lanes, even unused.
This patch fixes copy to only cover used lanes.
Differential Revision: https://reviews.llvm.org/D29105