[InstSimplify] fold select with implied condition

This is an almost direct move of the functionality from InstCombine to 
InstSimplify. There's no reason not to do this in InstSimplify because 
we never create a new value with this transform.

(There's a question of whether any dominance-based transform belongs in
either of these passes, but that's a separate issue.)

I've changed 1 of the conditions for the fold (1 of the blocks for the 
branch must be the block we started with) into an assert because I'm not 
sure how that could ever be false.

We need 1 extra check to make sure that the instruction itself is in a
basic block because passes other than InstCombine may be using InstSimplify
as an analysis on values that are not wired up yet.

The 3-way compare changes show that InstCombine has some kind of 
phase-ordering hole. Otherwise, we would have already gotten the intended
final result that we now show here.

llvm-svn: 347896