[SPIR-V] Ensure that we don't have a dangling BlockAddress constants after...

[SPIR-V] Ensure that we don't have a dangling BlockAddress constants after internal intrinsic 'spv_switch' is processed (#92390)

After internal intrinsic 'spv_switch' is processed we need to delete
G_BLOCK_ADDR instructions that were generated to keep track of the
corresponding basic blocks. If we just delete G_BLOCK_ADDR instructions
with BlockAddress operands, this leaves their BasicBlock counterparts in
a "address taken" status. This would make AsmPrinter to generate a
series of unneeded labels of a `"Address of block that was removed by
CodeGen"` kind. This PR is to ensure that we don't have a dangling
BlockAddress constants by zapping the BlockAddress nodes, and only after
that proceed with erasing G_BLOCK_ADDR instructions.

See also https://github.com/llvm/llvm-project/pull/87823 for more
details.