[handshake-runner] possible race condition when gathering the simulation time

I'm building on OSX and using Clang version 12, using a unified build and LLVM_USE_SANITIZER:STRING=Address;Undefined. I did a fresh build from scratch today and started seeing this issue.

Many of the handshake-runner tests are spuriously failing due to an error detected by UBSan:

andrewy@ayoung-0504~/w/c/build> /Users/andrewy/wsp/circt/build/bin/circt-opt -create-dataflow /Users/andrewy/wsp/circt/test/handshake-runner/histogram-std.mlir | /Users/andrewy/wsp/circt/build/bin/handshake-runner
0
/Users/andrewy/wsp/circt/tools/handshake-runner/Simulation.cpp:716:25: runtime error: 9.59553e+210 is outside the range of representable values of type 'int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /Users/andrewy/wsp/circt/tools/handshake-runner/Simulation.cpp:716:25 in
PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backtrace.
Stack dump:
0.	Program arguments: /Users/andrewy/wsp/circt/build/bin/handshake-runner
fish: Process 98048, '/Users/andrewy/wsp/circt/build/bin/handshake-runner' from job 1, '/Users/andrewy/wsp/circt/build/…' terminated by signal SIGABRT (Abort)

The code in question is here: https://github.com/llvm/circt/blob/main/tools/handshake-runner/Simulation.cpp#L715. When the double named time is larger than an integer can represent, UBSan is aborting the program. I added a printf:

llvm::errs() << "time: " << time << "\N";

right before the aborting line of code, and it looks like each test is 50% of the time printing some stable number (1.150000e+02 for the test above) and 50% of the time printing something very random looking which crashes.

Start with tools/handshake-runner/Simulation.cpp around line 715 and reproduce the failure using the sanitizer-enabled build and the histogram-std.mlir pipeline shown in the report. Compare the stable and random simulation-time results while investigating the reported race condition; done means handshake-runner tests no longer trigger UBSan or abort on an invalid time conversion.