don't bother calling ConstantFoldInstruction unless there is a use of the

don't bother calling ConstantFoldInstruction unless there is a use of the
instruction (which disqualifies stores, unreachable, etc) and at least the
first operand is a constant.  This filters out a lot of obvious cases that
can't be folded.  Also, switch the IRBuilder to a TargetFolder, which tries
harder.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@84170 91177308-0d34-0410-b5e6-96231b3b80d8