[BOLT][NFC] Print block instructions in dumpGraph as part of node label

Reuse the option `-dot-tooltip-code` to put block instructions into the label.
This way, the instructions are displayed by default when used with dot viewer.

When the .dot file is used with dot2html, instructions are hidden by default,
and are shown by clicking on a node.

{F23169510}

Reviewed By: rafauler

Differential Revision: https://reviews.llvm.org/D126237