Typing icount at the sim debug prompt will print the current icount.
si <mnemonic> steps through instructions until an instruction with that
mnemonic is seen.
E.g. si brc will stop at the next brc instruction before executing it.

R=jyan@ca.ibm.com, joransiu@ca.ibm.com

Review-Url: https://codereview.chromium.org/1944913002
Cr-Commit-Position: refs/heads/master@{#35997}