Before this commit all code relating to handling arguments in SILBasicBlock had
somewhere in the name BB. This is redundant given that the class's name is
already SILBasicBlock. This commit drops those names.
Some examples:
getBBArg() => getArgument()
BBArgList => ArgumentList
bbarg_begin() => args_begin()
For details see the comment in ConditionForwarding.cpp.
This optimization pass helps to optimize loops iterating over closed ranges, e.g. for i in 0...n { }
