[SystemZ] Provide basic TargetTransformInfo implementation

[SystemZ] Provide basic TargetTransformInfo implementation

This hooks up the TargetTransformInfo machinery for SystemZ,
and provides an implementation of getIntImmCost.

In addition, the patch adds the isLegalICmpImmediate and
isLegalAddImmediate TargetLowering overrides, and updates
a couple of test cases where we now generate slightly
better code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233688 91177308-0d34-0410-b5e6-96231b3b80d8