[SCCP] Remove manual folding of terminator instructions.

Summary:
BranchInst, SwitchInst (with non-default case) with Undef as input is not
possible at this point. As we always default-fold terminator to one target in
ResolvedUndefsIn and set the input accordingly.

So we should only have constantint/blockaddress here.

If ConstantFoldTerminator fails, that could mean 2 things.

1. ConstantFoldTerminator is doing something unexpected, i.e. not folding on constantint
or blockaddress and not making blocks that should be dead dead.
2. This is not a terminator on constantint or blockaddress. Its on a constant or
overdefined, then this block should not be dead.

In both cases, we should assert.

Reviewers: davide, efriedma, sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D30381

llvm-svn: 296281