XCore target: add XCoreTargetLowering::isZExtFree()

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192431 91177308-0d34-0410-b5e6-96231b3b80d8
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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192432 91177308-0d34-0410-b5e6-96231b3b80d8