[ScopBuilder] Simplify main statement flag in buildEqivClassBlockStmts. NFC.
When reading code in ScopBuilder::buildEqivClassBlockStmts, I think the
main statement flag computation can be simplified, here is the patch.
It's based on two simple facts that:
- Instruction won't be removed once it's inserted into UnionFind.
- Main statement must be set if there is non-trivial statement besides the last one.
The patch also saves std::find call.
Patch by bin.narwal <email@example.com>
Differential Revision: https://reviews.llvm.org/D66477