[LICM] Try to merge debug locations when sinking.

The current strategy LICM uses when sinking for debuginfo is
that of picking the debug location of one of the uses.
This causes stepping to be wrong sometimes, see, e.g. PR45523.

This patch introduces a generalization of getMergedLocation(),
that operates on a vector of locations instead of two, and try
to merge all them together, and use the new API in LICM.

<rdar://problem/61750950>