Introduce a new variant type 'TryResult' to represent the result of

TryEvaluateBool instead of using a raw 'int'.  This avoids any
confusion of how 'int' converts to bool, and makes the resultant code
easier to read.

Condense a bunch of 'addSuccessor()' calls in 'if ... else' to use the
ternary operator instead.

llvm-svn: 76947