JIT: Allow initializing blocks without jump targets

[amanasifkhalid]

...and remove BasicBlock::bbTempJumpDest, per discussion here in #93415. We still assert the jump target is set appropriately whenever it is read/written, and in the majority of cases, we still initialize blocks with their jump kind and target set simultaneously. This change improves usability for the few edge cases (like in Compiler::impImportLeave) where a block's jump target isn't known at initialization.

[ghost]

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

Issue Details

---

...and remove BasicBlock::bbTempJumpDest, per discussion here in #93415. We still assert the jump target is set appropriately whenever it is read/written, and in the majority of cases, we still initialize blocks with their jump kind and target set simultaneously. This change improves usability for the few edge cases (like in Compiler::impImportLeave) where a block's jump target isn't known at initialization.

Author:	amanasifkhalid
Assignees:	amanasifkhalid
Labels:	area-CodeGen-coreclr
Milestone:	-

[amanasifkhalid]

CC @dotnet/jit-contrib, @AndyAyersMS @jakobbotsch PTAL. I decided to implement Jakob's suggestion of not doing checks during block initialization, and instead performing them when reading/writing the block's jump target. To do this, I had to refactor Compiler::bbNewBasicBlock to be part of the BasicBlock struct so it can directly modify the jump target members without using the modifier methods that contain asserts.

No diffs.

[amanasifkhalid]

Failure looks like #48798

[AndyAyersMS]

Looks ok to me, but I'll let @jakobbotsch approve if he's ok with this.

[jakobbotsch]

LGTM. Nit: now that the function is part of BasicBlock you might consider just naming it BasicBlock::New or something of the sort (and if you decide to do that, feel free to do it in a follow-up to avoid rerunning CI again)

[amanasifkhalid]

(and if you decide to do that, feel free to do it in a follow-up to avoid rerunning CI again)

I'll include the rename in the next PR. Thanks!