[IR] Share implementation of pairs of const and non-const methods in...

[IR] Share implementation of pairs of const and non-const methods in BasicBlock using the const version instead of the non-const version

Summary:
During post-commit review of a previous change I made it was pointed out that const casting 'this' is technically a bad practice. This patch re-implements all of the methods in BasicBlock that do this to use the const BasicBlock version and const_cast the return value instead.

I think there are still many other classes that do similar things. I may look at more in the future.

Reviewers: dblaikie

Reviewed By: dblaikie

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31377

llvm-svn: 298827