[SPIR-V] Re-implement switch and improve validation of forward calls (#87823)

This PR fixes issue https://github.com/llvm/llvm-project/issues/87763
and preserves valid CFG in cases when previous scheme failed to generate
valid code for a switch statement. The PR hardens one existing test case
and adds one more test case as a validation of a new switch generation.
Tests are passing spirv-val now.

This PR also improves validation of forward calls.