Specify branch probabilities for callbr dests

Summary:
callbr's indirect branches aren't expected to be taken, so reduce their
probabilities to 0 while increasing the default destination to 1. This
allows some code improvements through block placement.

Reviewers: nickdesaulniers

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72656