compiler/spirv: reject invalid shader code properly

compiler/spirv: reject invalid shader code properly

After bebe3d626e5, b->fail_jump is prepared after vtn_create_builder
which can longjmp(3) to it through its vtx_assert()s.  This corrupts
the stack and creates confusing core dumps, so we need to avoid it.

While there, I decided to print the offending values for debugability.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>