Rename BasicBlock and StmtBlock to Block, and make a pass cleaning it up. I...

Rename BasicBlock and StmtBlock to Block, and make a pass cleaning it up.  I did not make an effort to rename all of the 'bb' names in the codebase, since they are still correct and any specific missed once can be fixed up on demand.

The last major renaming is Statement -> Instruction, which is why Statement and
Stmt still appears in various places.

This is step 19/n towards merging instructions and statements, NFC.

PiperOrigin-RevId: 227163082