[Constant] Inline ConstantInt::getSigned

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
the end.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D146598