[LiveIntervals] Handle moving up dead partial write

Summary:
In the test case, the machine scheduler moves a dead write to a subreg
up into the middle of a segment of the overall reg's live range, where
the segment had liveness only for other subregs in the reg.
handleMoveUp created an invalid live range, causing an assert a bit
later.

This commit fixes it to handle that situation. The segment is split in
two at the insertion point, and the part after the split, and any
subsequent segments up to the old position, are changed to be defined by
the moved def.

V2: Better test.

Subscribers: MatzeB, nhaehnle, llvm-commits

Differential Revision: https://reviews.llvm.org/D43478

Change-Id: Ibc42445ddca84e79ad1f616401015d22bc63832e
llvm-svn: 326087