Speculatively execute a block when the the block is the then part of a...

Speculatively execute a block when the the block is the then part of a triangle shape and it contains a single, side effect free, cheap instruction. The branch is eliminated by adding a select instruction. i.e.

Turn                                                                                                                                                                                                       
BB:                                                                                                                                                                                                        
    %t1 = icmp                                                                                                                                                                                             
    br i1 %t1, label %BB1, label %BB2                                                                                                                                                                      
BB1:                                                                                                                                                                                                       
    %t3 = add %t2, c                                                                                                                                                                                       
    br label BB2                                                                                                                                                                                           
BB2:                                                                                                                                                                                                       
=>                                                                                                                                                                                                         
BB:                                                                                                                                                                                                        
    %t1 = icmp                                                                                                                                                                                             
    %t4 = add %t2, c                                                                                                                                                                                       
    %t3 = select i1 %t1, %t2, %t3

llvm-svn: 52073