Replace TargetLowering::isIntImmLegal() with

Replace TargetLowering::isIntImmLegal() with
ScalarTargetTransformInfo::getIntImmCost() instead. "Legal" is a poorly defined
term for something like integer immediate materialization. It is always possible
to materialize an integer immediate. Whether to use it for memcpy expansion is
more a "cost" conceern.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169929 91177308-0d34-0410-b5e6-96231b3b80d8