Use a loop to simplify the runtime unrolling prologue.

Runtime unrolling will create a prologue to execute the extra
iterations which is can't divided by the unroll factor. It
generates an if-then-else sequence to jump into a factor -1
times unrolled loop body, like

    extraiters = tripcount % loopfactor
    if (extraiters == 0) jump Loop:
    if (extraiters == loopfactor) jump L1
    if (extraiters == loopfactor-1) jump L2
    ...
    L1:  LoopBody;
    L2:  LoopBody;
    ...
    if tripcount < loopfactor jump End
    Loop:
    ...
    End:

It means if the unroll factor is 4, the loop body will be 7
times unrolled, 3 are in loop prologue, and 4 are in the loop.
This commit is to use a loop to execute the extra iterations
in prologue, like

        extraiters = tripcount % loopfactor
        if (extraiters == 0) jump Loop:
        else jump Prol
 Prol:  LoopBody;
        extraiters -= 1                 // Omitted if unroll factor is 2.
        if (extraiters != 0) jump Prol: // Omitted if unroll factor is 2.
        if (tripcount < loopfactor) jump End
 Loop:
 ...
 End:

Then when unroll factor is 4, the loop body will be copied by
only 5 times, 1 in the prologue loop, 4 in the original loop.
And if the unroll factor is 2, new loop won't be created, just
as the original solution.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218604 91177308-0d34-0410-b5e6-96231b3b80d8