Fix binary spans on wrapper blocks in IRBuilder

[tlively]

When a parsed module contains branches to control flow structures other than blocks or loops, IRBuilder must create a wrapper block around the control flow structure for those branches to target. When that wrapper block is pushed onto the stack, the current binary position is attached as its end position. But the matching start position was previously still attached to the original control flow structure. The lack of an end position on the original control flow structure could cause an assertion failure later when then debug info was written back out.

Fix the bug by moving the start position from the original control flow structure to the wrapper block where the matching end position will be attached. Update printing of code offsets to include the end offset to make it more obvious in the test that the bug is fixed.

Fixes #8940.

[tlively]

Any thoughts about how to test this?

[kripken]

For testing, you can use BINARYEN_PRINT_FULL=1 to print out these wrapper blocks in the IR, and their debug info. Though if my comment above makes sense, that might be moot.

[kripken]

Also, can we perhaps document this wrapping process somewhere? That neither AI nor I could figure it out in a reasonable time is unfortunate, as I wrote in the other issue.