This change fixed a bug in the function visitMul.  The prior version
assumed that a constant on the RHS of a multiplication was either an
IntConstant or an FPConstant.  It checked for an IntConstant and then,
if it did not find one, did a hard cast to an FPConstant.  That code
would crash if the RHS were a ConstantExpr that was neither an
IntConstant nor an FPConstant.  This version replaces the hard cast
with a dyn_cast.  It performs the same way for IntConstants and
FPConstants but does nothing, instead of crashing, for constant
expressions.

The regression test for this change is 2004-07-27-ConstantExprMul.ll.

llvm-svn: 15291