Always populate m_constraints.typeSrcs for LdLocs added by insertLocalPhis

Summary: It wasn't adding an entry into the m_constraints.typeSrcs map when the
set was empty.

Also, dump the unit in IRBuilder::constrainValue if
m_constraints.typeSrcs doesn't have an entry for an instruction.

Reviewed By: @bertmaher

Differential Revision: D1559672