JIT: Tail-merge process all sets at once

[BoyBaykiller]

Say we call tailMergePreds() with candidates: [A1, B2, C1, A3, B3, B1, A2].

Previously, it only processed one set at a time - let's say all the A's.
Then we'd regather all candidates in the caller, skipping the ones which where already processed with checks like
!block->KindIs(BBJ_RETURN, BBJ_THROW) || block->isEmpty() and call it again until it finds no more.

With this PR all sets are processed in a single call to tailMergePreds.
It works as follows. We start at index 0 (A1) find the matches (all the other A's) and make them continous in memory, so we get: [A1, A3, A2, B2, C1, B3, B1] after the first call to partition. We then advance by the number of matches (3) and continue at B2. After the next call to partition we get: [A1, A3, A2, B2, B3, B1, C1].

I didn't use the STL std::partiton, as discussed.

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[BoyBaykiller]

@AndyAyersMS PTAL. The diffs are what I expected, with the exception of the tp regression on x86 - I have no clue why that is and I don't know a good way of finding out.

The tiny assembly diffs are only because I also moved return/throw merging to happen before tail-merge and sometimes the crossJumpVictim which would have been picked is now removed/empty.