Spill mode: Hoist back-copies locally.

Spill mode: Hoist back-copies locally.

The leaveIntvAfter() function normally inserts a back-copy after the
requested instruction, making the back-copy kill the live range.

In spill mode, try to insert the back-copy before the last use instead.
That means the last use becomes the kill instead of the back-copy.  This
lowers the register pressure because the last use can now redefine the
same register it was reading.

This will also improve compile time: The back-copy isn't a kill, so
hoisting it in hoistCopiesForSize() won't force a recomputation of the
source live range.  Similarly, if the back-copy isn't hoisted by the
splitter, the spiller will not attempt hoisting it locally.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@139883 91177308-0d34-0410-b5e6-96231b3b80d8