[Bitcode] Drop invalid branch_weight in BitcodeReader

Summary:
If bitcode reader gets an invalid branch weight, drop that from the
inputs. This allows us to read the broken modules we generated before
the verifier was able to catch this.

rdar://64870641

Reviewers: yrouban, t.p.northover, dexonsmith, arphaman, aprantl

Reviewed By: aprantl

Subscribers: aprantl, hiraditya, jkorous, ributzka, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83699