[CodeGen] Add invalidation of the loop SCEVs after merge block generation.

The SCEVs of loops surrounding the escape users of a merge blocks are
forgotten, so that loop trip counts based on old values can be revoked.

This fixes llvm.org//PR32536

Contributed-by: Baranidharan Mohan <mbdharan@gmail.com>

Differential Revision: https://reviews.llvm.org/D33195

llvm-svn: 303561