Verifier: Avoid quadratic checking of aggregates for bad bitcasts

Avoid O(N^2) behaviour when checking for bad bitcasts in `ConstantExpr`s
buried inside of aggregate initializers to `GlobalVariable`s.  I've:
- centralized the "visited" set for recursing through `ConstantExpr`s so
  that expressions are only visited once per Verifier run,
- removed the duplicate logic for the stack visit, and
- avoided recursing into other `GlobalValue`s.

This recovers roughly a 100x time difference in clang compiles of a
particular input file (filled with large cross-referencing tables) that
depends on whether `-disable-llvm-verifier` is on.  This slowdown was
caused by r187506, which introduced these checks.

Now, avoiding `-disable-llvm-verifier` only causes a 2x slowdown for
this case.

(Interestingly, dumping the textual IR for this file starts at least
50GB of global variable initializers (I don't know the total, since I
killed the dump)...)

llvm-svn: 255269