[CFG/BasicBlock] Rename pred_const to const_pred. [NFC]

Summary:
Rename `pred_const_range` to `const_pred_range` to make it consistent with
the other pred/succ iterator definitions.

Reviewers: nicholas, dblaikie, nlewycky

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75962