[SCCP] Don't use markForcedConstant on branch conditions.

It's more aggressive than we need to be, and leads to strange
workarounds in other places like call return value inference. Instead,
just directly mark an edge viable.

Tests by Florian Hahn.

Differential Revision: https://reviews.llvm.org/D49408



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@337507 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed