Distinguish complex mapped values from forced recomputation.

When a ParentVNI maps to multiple defs in a new interval, its live range
may still be derived directly from RegAssign by transferValues().

On the other hand, when instructions have been rematerialized or
hoisted, it may be necessary to completely recompute live ranges using
LiveRangeCalc::extend() to all uses.

Use a bit in the value map to indicate that a live range must be
recomputed.  Rename markComplexMapped() to forceRecompute().

This fixes some live range verification errors when
-split-spill-mode=size hoists back-copies by recomputing source ranges
when RegAssign kills can't be moved.

llvm-svn: 139660