[NFC][AssemblyWriter] Allow AssemblyWriter::printBasicBlock() to print blocks that...

[NFC][AssemblyWriter] Allow AssemblyWriter::printBasicBlock() to print blocks that don't have parents.

Remove the assert in AssemblyWriter::printBasicBlock() and
in BasicBlock::isEntryBlock() that require blocks to have parents.
Instead, have BasicBlock::isEntryBlock() return false for unattached
blocks. This allows us to call these functions for blocks that are
not yet added to a module which is a useful debugging capability.

Committing for xiaoqing_wu

https://reviews.llvm.org/D106127k