rdar://106182518

Currently, in control flow instructions with multiple exits (namely br_if and br_table), we assume that the calling convention for each target’s block arguments will be the same. This is not the case universally though: 
 - When targeting the top level block, the results are allocated according to the WASM calling convention, not the block calling convention.
 - If we run out of registers to use for the block calling convention, we pass arguments through WASM canonical stack slots. But the stack height of these may vary between target blocks, even if they have the same signature.

To fix this, we need to modify the ControlData methods currently used to manage control flow blocks, to permit setting up for multiple distinct exit branches per block.