[spirv] Fix function entry block erase after moving to spv.selection

[spirv] Fix function entry block erase after moving to spv.selection

The structured selection/loop's entry block does not have arguments.
If the function's header block is also part of the structured control
flow, we cannot just simply erase it because it may contain arguments
matching the function signature and used by the cloned blocks. Instead,
turn it into a block only containing a spv.Branch op.

Also, we can directly emit instructions for the spv.selection header
block to the block containing the spv.selection op. This eliminates
unnecessary branches in the SPIR-V blob.

Added a test for nested spv.loop.

PiperOrigin-RevId: 273351424