XCore target: add XCoreTargetLowering::isZExtFree()
XCore target: fix bug in XCoreLowerThreadLocal.cpp

When a ConstantExpr which uses a thread local is part of a PHI node
instruction, the insruction that replaces the ConstantExpr must
be inserted in the predecessor block, in front of the terminator instruction.
If the predecessor block has multiple successors, the edge is first split.