Ensure backwards jumps get a yk_location.

[vext01]

Requires: ykjit/yk#1466

I noticed that the mandelbrot benchmark was not triggering the JIT. This was because we only assigned locations to for loops, but this benchmark contains mostly while looping.

Lua has no deidcated bytecode for while loops. Instead it uses unstructured control flow using conditionals and jumps.

Here's an example:

i = 10
while i > 0 do
    i = i - 1
end

Becomes:

main <a.lua:0,0> (11 instructions at 0xbfc160)
0+ params, 2 slots, 1 upvalue, 0 locals, 2 constants, 0 functions
        1       [1]     VARARGPREP      0
        2       [1]     SETTABUP        0 0 1k  ; _ENV "i" 10
        3       [2]     GETTABUP        0 0 0   ; _ENV "i"
        4       [2]     GTI             0 0 0
        5       [2]     JMP             5       ; to 11
        6       [3]     GETTABUP        0 0 0   ; _ENV "i"
        7       [3]     ADDI            0 0 -1
        8       [3]     MMBINI          0 1 7 0 ; __sub
        9       [3]     SETTABUP        0 0 0   ; _ENV "i"
        10      [3]     JMP             -8      ; to 3
        11      [4]     RETURN          0 1 1   ; 0 out

In this example JMP -8 is a CFG back edge that makes the loop.

This commit makes yklua recognise back edges as potential loops so that they get JITted if/when they get hot. This is of course a proxy: not all back edges are necessarily loops, but it should (tm) be a decent enough approximation for our purposes.

I measured about a 12% slowdown with this change. We can't not trace while loops and expect to be faster, so we will have to take this on for now.

[vext01]

Once ykjit/yk#1469 is merged, this can go in too.

[ltratt]

Right now this is fine but it's probably not what we want in the medium term because it introduces several sort-of-constant comparisons that will have an overhead on the interpreter (probably the JIT removes all of this).

We've got at least two possible ways here:

1. Rely on yklocs[pc] being NULL/non-NULL for the right opcodes. Downside: we have to do a load for every instruction (which might obliterate any advantage we get from removing the comparisons).
2. Is there a spare bit in an opcode we can reuse for "this particular opcode should have a non-null YkLocation"?

[vext01]

Honestly, I just did the smallest change possible to get while loops traced.

We used to do 1., but that seems to have been changed at some point:

yklua/src/lvm.c

Line 1181 in 51b872e

YkLocation *ykloc = yk_lookup_ykloc(ci, pc);

2. I couldn't say without studying lua more.