JIT: Tail-merge don't produce empty blocks

[BoyBaykiller]

I had an other change for improving the crossJumpVictim selection logic in tail-merge, but it had random regressions because downstream phases get confused depending on where empty blocks are. This PR stops producing empty blocks in tail-merge which fixes that.
It also makes sense given how we are running directly after "optimize control flow" phase which just got rid of all empty blocks.

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[BoyBaykiller]

This itself has some interesting regressions related to how the backend deals with the block layout.
Particularly arround bbWeight=0 and BBJ_RETURN it seems, but haven't looked deeply.

I see cases like this where we get weird unnecessary jumps.

G_M48648_IG02:  ;; offset=0x0000
       vucomiss xmm0, xmm1
       jp       SHORT G_M48648_IG03
       je       SHORT G_M48648_IG08
						;; size=8 bbWeight=1 PerfScore 4.00
G_M48648_IG03:  ;; offset=0x0008
       vucomiss xmm0, xmm0
       jp       SHORT G_M48648_IG04
       vucomiss xmm0, xmm1
       jbe      SHORT G_M48648_IG07
						;; size=12 bbWeight=1 PerfScore 6.00
G_M48648_IG04:  ;; offset=0x0014
       vmovaps  xmm1, xmm0
						;; size=4 bbWeight=1 PerfScore 0.25
G_M48648_IG05:  ;; offset=0x0018
       vmovaps  xmm0, xmm1
						;; size=4 bbWeight=1 PerfScore 0.25
G_M48648_IG06:  ;; offset=0x001C
       ret      
						;; size=1 bbWeight=1 PerfScore 1.00
G_M48648_IG07:  ;; offset=0x001D
       jmp      SHORT G_M48648_IG05
						;; size=2 bbWeight=0 PerfScore 0.00
G_M48648_IG08:  ;; offset=0x001F
       vmovd    eax, xmm1
       test     eax, eax
       jl       SHORT G_M48648_IG04
       jmp      SHORT G_M48648_IG07
						;; size=10 bbWeight=0 PerfScore 0.00

[BoyBaykiller]

@AndyAyersMS PTAL.

[AndyAyersMS]

Do we get similar results if we swap the order of the second tail merge pass with the flow graph opts? Or is this enabling more merging?

[BoyBaykiller]

I've tried moving PHASE_OPTIMIZE_FLOW after PHASE_HEAD_TAIL_MERGE2 and also tried doTailDuplication: false, but it had bad diffs.
Mayeb it's intentional that PHASE_OPTIMIZE_FLOW expands out all tails and PHASE_HEAD_TAIL_MERGE2 then merges them.