[BOLT][NFC] Use colors in CFG dumps

Use color coding to distinguish nodes:
- Entry nodes have bold border
- Scalar (non-loopy) code is milk white
- Outer loops are light yellow
- Innermost loops are light blue

`-print-loops` needs to be enabled to provide BinaryLoopInfo.
Examples:
{F23170673}
{F23170680}

Reviewed By: rafauler

Differential Revision: https://reviews.llvm.org/D126248