[StringExtras] Add a helper class for comma-separated lists
This patch introduces a helper class SubsequentDelim to simplify loops
that generate a comma-separated lists.
For example, consider the following loop, taken from
llvm/lib/CodeGen/MachineBasicBlock.cpp:
for (auto I = pred_begin(), E = pred_end(); I != E; ++I) {
if (I != pred_begin())
OS << ", ";
OS << printMBBReference(**I);
}
The new class allows us to rewrite the loop as:
SubsequentDelim SD;
for (auto I = pred_begin(), E = pred_end(); I != E; ++I)
OS << SD << printMBBReference(**I);
where SD evaluates to the empty string for the first time and ", " for
subsequent iterations.
Unlike interleaveComma, defined in llvm/include/llvm/ADT/STLExtras.h,
SubsequentDelim can accommodate a wider variety of loops, including:
- those that conditionally skip certain items,
- those that need iterators to call getSuccProbability(I), and
- those that iterate over integer ranges.
As an example, this patch cleans up MachineBasicBlock::print.
Differential Revision: https://reviews.llvm.org/D94377