[VPlan] Script to extract VPlan digraphs from log

The vectoriser's debug log prints VPlan digraphs, but it's a bit
cumbersome to extract them and render them into PNG images. This script
does exactly that, being careful enough to extract all individual plans,
name them appropriately and save in either .dot or .png files.

Example usage:

$ opt -O3 -debug-only=loop-vectorize file.ll -S -o /dev/null 2> debug.log

$ $LLVM_SRC/utils/extract_vplan.py < debug.log
Exporting VF1UF1 to DOT: VPlanVF1UF1.dot
Exporting VF24UF1 to DOT: VPlanVF24UF1.dot

$ $LLVM_SRC/utils/extract_vplan.py --png < debug.log
Exporting VF1UF1 to PNG via dot: VPlanVF1UF1.png
Exporting VF24UF1 to PNG via dot: VPlanVF24UF1.png

$ xdot VPlanVF1UF1.dot

Differential Revision: https://reviews.llvm.org/D53142

llvm-svn: 344599