Implement a few symbolic constant folding things. X ? Y : Y is Y.

Fold:
seteq ({ short }* cast (int 1 to { short }*), { short }* null)
setlt ({ short }* cast (int 1 to { short }*), { short }* cast (int 2 to { short }*))

to false/true.  These last two commonly occur in the output of compilers that
tag integers, like cozmic's scheme compiler.

Tested by Regression/Assembler/ConstantExprFold.llx

llvm-svn: 25112