spirv-val: Add test for Conditional Branch without an exit being valid

[cmarcelo]

This patch is mutual exclusive with #5069.

[Keenuts]

Not a SCFG/CFG expert at all, but I'd say this should compile?

• I'm assuming Undef doesn't mean a block is unreachable, just we have an undefined value, so branch decision will be implementation defined.
• From the SPIR-V spec around SCGF, it seems the structure is OK, and we are not breaking rules around loop structure, as we have an exit which seem structurally OK.

[cmarcelo]

Not a SCFG/CFG expert at all, but I'd say this should compile?

* I'm assuming Undef doesn't mean a block is unreachable, just we have an undefined value, so branch decision will be implementation defined.

* From the SPIR-V spec around SCGF, it seems the structure is OK, and we are not breaking rules around loop structure, as we have an exit which seem structurally OK.

I was unsure whether the OpBranchConditional would require an OpSelectionMerge in this case. Upon discussing with others it seems this is covered by:

Selections must be structured. That is, an OpSelectionMerge instruction is required to precede:

    an OpSwitch instruction

    an OpBranchConditional instruction that has different True Label and False Label operands where neither are declared merge blocks or Continue Targets.

so would make this invalid SPIR-V.

I'm closing this in favor of #5069.