CodeGen: If Convert blocks that would form a diamond when tail-merged.

The following function currently relies on tail-merging for if
conversion to succeed. The common tail of cond_true and cond_false is
extracted, and this then forms a diamond pattern that can be
successfully if converted.

If this block does not get extracted, either because tail-merging is
disabled or the threshold is higher, we should still recognize this
pattern and if-convert it.
define i32 @t2(i32 %a, i32 %b) nounwind {
entry:
	%tmp1434 = icmp eq i32 %a, %b		; <i1> [#uses=1]
	br i1 %tmp1434, label %bb17, label %bb.outer

bb.outer:		; preds = %cond_false, %entry
	%b_addr.021.0.ph = phi i32 [ %b, %entry ], [ %tmp10, %cond_false ]
	%a_addr.026.0.ph = phi i32 [ %a, %entry ], [ %a_addr.026.0, %cond_false ]
	br label %bb

bb:		; preds = %cond_true, %bb.outer
	%indvar = phi i32 [ 0, %bb.outer ], [ %indvar.next, %cond_true ]
	%tmp. = sub i32 0, %b_addr.021.0.ph
	%tmp.40 = mul i32 %indvar, %tmp.
	%a_addr.026.0 = add i32 %tmp.40, %a_addr.026.0.ph
	%tmp3 = icmp sgt i32 %a_addr.026.0, %b_addr.021.0.ph
	br i1 %tmp3, label %cond_true, label %cond_false

cond_true:		; preds = %bb
	%tmp7 = sub i32 %a_addr.026.0, %b_addr.021.0.ph
	%tmp1437 = icmp eq i32 %tmp7, %b_addr.021.0.ph
	%indvar.next = add i32 %indvar, 1
	br i1 %tmp1437, label %bb17, label %bb

cond_false:		; preds = %bb
	%tmp10 = sub i32 %b_addr.021.0.ph, %a_addr.026.0
	%tmp14 = icmp eq i32 %a_addr.026.0, %tmp10
	br i1 %tmp14, label %bb17, label %bb.outer

bb17:		; preds = %cond_false, %cond_true, %entry
	%a_addr.026.1 = phi i32 [ %a, %entry ], [ %tmp7, %cond_true ], [ %a_addr.026.0, %cond_false ]
	ret i32 %a_addr.026.1
}

Without tail-merging or diamond-tail if conversion:
LBB1_1:                                 @ %bb
                                        @ =>This Inner Loop Header: Depth=1
        cmp     r0, r1
        ble     LBB1_3
@ BB#2:                                 @ %cond_true
                                        @   in Loop: Header=BB1_1 Depth=1
        subs    r0, r0, r1
        cmp     r1, r0
        it      ne
        cmpne   r0, r1
        bgt     LBB1_4
LBB1_3:                                 @ %cond_false
                                        @   in Loop: Header=BB1_1 Depth=1
        subs    r1, r1, r0
        cmp     r1, r0
        bne     LBB1_1
LBB1_4:                                 @ %bb17
        bx      lr

With diamond-tail if conversion, but without tail-merging:
@ BB#0:                                 @ %entry
        cmp     r0, r1
        it      eq
        bxeq    lr
LBB1_1:                                 @ %bb
                                        @ =>This Inner Loop Header: Depth=1
        cmp     r0, r1
        ite     le
        suble   r1, r1, r0
        subgt   r0, r0, r1
        cmp     r1, r0
        bne     LBB1_1
@ BB#2:                                 @ %bb17
        bx      lr

llvm-svn: 277905