Fix miscompile due to StackColoring incorrectly merging stack slots (PR15707)

IR optimisation passes can result in a basic block that contains:

  llvm.lifetime.start(%buf)
  ...
  llvm.lifetime.end(%buf)
  ...
  llvm.lifetime.start(%buf)

Before this change, calculateLiveIntervals() was ignoring the second
lifetime.start() and was regarding %buf as being dead from the
lifetime.end() through to the end of the basic block.  This can cause
StackColoring to incorrectly merge %buf with another stack slot.

Fix by removing the incorrect Starts[pos].isValid() and
Finishes[pos].isValid() checks.

Just doing:
      Starts[pos] = Indexes->getMBBStartIdx(MBB);
      Finishes[pos] = Indexes->getMBBEndIdx(MBB);
unconditionally would be enough to fix the bug, but it causes some
test failures due to stack slots not being merged when they were
before.  So, in order to keep the existing tests passing, treat LiveIn
and LiveOut separately rather than approximating the live ranges by
merging LiveIn and LiveOut.

This fixes PR15707.
Patch by Mark Seaborn.