Throttle back tail duplication to avoid creating really ugly sequences of code.

Throttle back tail duplication to avoid creating really ugly sequences of code.
For Transforms/TailDup/if-tail-dup.ll, f.e., it produces:

_foo:
        movl 8(%esp), %eax
        movl 4(%esp), %ecx
        testl $1, %ecx
        je LBB1_2       #cond_next
LBB1_1: #cond_true
        movl $1, (%eax)
LBB1_2: #cond_next
        testl $2, %ecx
        je LBB1_4       #cond_next10
LBB1_3: #cond_true6
        movl $1, 4(%eax)
LBB1_4: #cond_next10
        testl $4, %ecx
        je LBB1_6       #cond_next18
LBB1_5: #cond_true14
        movl $1, 8(%eax)
LBB1_6: #cond_next18
        testl $8, %ecx
        je LBB1_8       #return
LBB1_7: #cond_true22
        movl $1, 12(%eax)
        ret
LBB1_8: #return
        ret

instead of:

_foo:
        movl 4(%esp), %eax
        testl $2, %eax
        sete %cl
        movl 8(%esp), %edx
        testl $1, %eax
        je LBB1_2       #cond_next
LBB1_1: #cond_true
        movl $1, (%edx)
        testb %cl, %cl
        jne LBB1_4      #cond_next10
        jmp LBB1_3      #cond_true6
LBB1_2: #cond_next
        testb %cl, %cl
        jne LBB1_4      #cond_next10
LBB1_3: #cond_true6
        movl $1, 4(%edx)
        testl $4, %eax
        je LBB1_6       #cond_next18
        jmp LBB1_5      #cond_true14
LBB1_4: #cond_next10
        testl $4, %eax
        je LBB1_6       #cond_next18
LBB1_5: #cond_true14
        movl $1, 8(%edx)
        testl $8, %eax
        je LBB1_8       #return
        jmp LBB1_7      #cond_true22
LBB1_6: #cond_next18
        testl $8, %eax
        je LBB1_8       #return
LBB1_7: #cond_true22
        movl $1, 12(%edx)
        ret
LBB1_8: #return
        ret

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@30158 91177308-0d34-0410-b5e6-96231b3b80d8