ConstantInt::getSigned calls ConstantInt::get with the IsSigned flag set to true.
That flag normally defaults to false.
For always signed constants the code base is not consistent about whether
it uses ConstantInt::getSigned or ConstantInt::get with IsSigned set to true.
And it's not clear how to decide which way to use.
By making getSigned inline, both ways should generate the same code in