Add support for --debug-only=interpreter, to print out instrs before

interpreting them.

Move support for getting the value of a ConstantExpr into
getConstantExprValue(), and add support for the rest of the different
kinds of ConstantExprs.  (I don't think I like ConstantExprs!)
This requires separate procedures executeShlInst() and executeShrInst().

Reduce the number of references to TheEE.

Get rid of an old comment mentioning annotations.

Fix exitCalled(), which was crashing the Interpreter. This was a
leftover from the return-value code refactoring.

llvm-svn: 10389