[analyzer] Fix some expressions staying live too long. Add a debug checker.

StaticAnalyzer uses the CFG-based RelaxedLiveVariables analysis in order to,
in particular, figure out values of which expressions are still needed.
When the expression becomes "dead", it is garbage-collected during
the dead binding scan.

Expressions that constitute branches/bodies of control flow statements,
eg. `E1' in `if (C1) E1;' but not `E2' in `if (C2) { E2; }', were kept alive
for too long. This caused false positives in MoveChecker because it relies
on cleaning up loop-local variables when they go out of scope, but some of those
live-for-too-long expressions were keeping a reference to those variables.

Fix liveness analysis to correctly mark these expressions as dead.

Add a debug checker, debug.DumpLiveStmts, in order to test expressions liveness.

Differential Revision: https://reviews.llvm.org/D55566

llvm-svn: 349320