Fix a problem that Nate noticed that boils down to an over conservative check

in the code that does "select C, (X+Y), (X-Y) --> (X+(select C, Y, (-Y)))".
We now compile this loop:

LBB1_1: ; no_exit
        add r6, r2, r3
        subf r3, r2, r3
        cmpwi cr0, r2, 0
        addi r7, r5, 4
        lwz r2, 0(r5)
        addi r4, r4, 1
        blt cr0, LBB1_4 ; no_exit
LBB1_3: ; no_exit
        mr r3, r6
LBB1_4: ; no_exit
        cmpwi cr0, r4, 16
        mr r5, r7
        bne cr0, LBB1_1 ; no_exit

into this instead:

LBB1_1: ; no_exit
        srawi r6, r2, 31
        add r2, r2, r6
        xor r6, r2, r6
        addi r7, r5, 4
        lwz r2, 0(r5)
        addi r4, r4, 1
        add r3, r3, r6
        cmpwi cr0, r4, 16
        mr r5, r7
        bne cr0, LBB1_1 ; no_exit

llvm-svn: 26356