[MLIR] Rename collapsePLoops -> collapseParallelLoops

[tpopp]

Summary

Additionally, NFC code cleanups were done.

This is to address additional comments on
https://reviews.llvm.org/D76363

[bondhugula]

mlir/lib/Transforms/Utils/LoopUtils.cpp

1229

inner.getBody() to avoid confusion since this is a single block region.

[tpopp]

mlir/lib/Transforms/Utils/LoopUtils.cpp

1229

I'll block this on whatever happens with https://reviews.llvm.org/D77060

There's an annoying detail right now that constructing an OpBuilder on a Block vs Region create different insertion points (end() and begin()) respectively, so I'll revert this line and another below if it resolves in a different way, or do as you suggest here if it lands.

[tpopp]

mlir/lib/Transforms/Utils/LoopUtils.cpp

1229

This is now OpBuilder::atBlockBegin() in order to be more explicit.