If a function always returns a constant, replace all calls sites with that

If a function always returns a constant, replace all calls sites with that
constant value.  This makes the return value dead and allows for
simplification in the caller.

This implements IPConstantProp/return-constant.ll

This triggers several dozen times throughout SPEC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@17730 91177308-0d34-0410-b5e6-96231b3b80d8