Add options to dump block frequency/branch probability info in text.

Summary:
Add options -print-bfi/-print-bpi that dump block frequency and branch
probability info like -view-block-freq-propagation-dags and
-view-machine-block-freq-propagation-dags do but in text.

This is useful when the graph is very large and complex (the dot command
crashes, lines/edges too close to tell apart, hard to navigate without textual
search) or simply when text is preferred.

Reviewers: davidxl

Reviewed By: davidxl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37165

llvm-svn: 311822