Jit: fix some sources of x64 chk/rel diffs

[AndyAyersMS]

Break IGs for perfscores the same way in chk and rel. Also, fix the xarch
peephole to work across extended IGs.

Closes #39845.

[AndyAyersMS]

@briansull ptal
cc @dotnet/jit-contrib

No codegen diffs; gc info for release now matches that for chk.

There is at least still one more source of chk/rel diffs in I haven't tracked down; it shows up in pinvoke call prologs.

[AndyAyersMS]

Perf run failure is a CLI download error, will ignore.

[2020/07/24 08:23:51][INFO] DotNet Install Path: 'C:\h\w\A902095C\p\performance\tools\dotnet\x86'
[2020/07/24 08:23:51][INFO] Downloading https://dot.net/v1/dotnet-install.ps1
Traceback (most recent call last):
  File "C:\h\w\A902095C\p\performance\scripts\benchmarks_ci.py", line 264, in <module>
    __main(sys.argv[1:])
  File "C:\h\w\A902095C\p\performance\scripts\benchmarks_ci.py", line 208, in __main
    verbose=verbose
  File "C:\h\w\A902095C\p\performance\scripts\benchmarks_ci.py", line 75, in init_tools
    verbose=verbose,
  File "C:\h\w\A902095C\p\performance\scripts\dotnet.py", line 689, in install
    with urlopen(dotnetInstallScriptUrl) as response:
  File "C:\python3.7.0\lib\urllib\request.py", line 222, in urlopen
    return opener.open(url, data, timeout)
  File "C:\python3.7.0\lib\urllib\request.py", line 531, in open
    response = meth(req, response)
  File "C:\python3.7.0\lib\urllib\request.py", line 641, in http_response
    'http', request, response, code, msg, hdrs)
  File "C:\python3.7.0\lib\urllib\request.py", line 563, in error
    result = self._call_chain(*args)
  File "C:\python3.7.0\lib\urllib\request.py", line 503, in _call_chain
    result = func(*args)
  File "C:\python3.7.0\lib\urllib\request.py", line 755, in http_error_302
    return self.parent.open(new, timeout=req.timeout)
  File "C:\python3.7.0\lib\urllib\request.py", line 525, in open
    response = self._open(req, data)
  File "C:\python3.7.0\lib\urllib\request.py", line 543, in _open
    '_open', req)
  File "C:\python3.7.0\lib\urllib\request.py", line 503, in _call_chain
    result = func(*args)
  File "C:\python3.7.0\lib\urllib\request.py", line 1360, in https_open
    context=self._context, check_hostname=self._check_hostname)
  File "C:\python3.7.0\lib\urllib\request.py", line 1320, in do_open
    r = h.getresponse()
  File "C:\python3.7.0\lib\http\client.py", line 1321, in getresponse
    response.begin()
  File "C:\python3.7.0\lib\http\client.py", line 296, in begin
    version, status, reason = self._read_status()
  File "C:\python3.7.0\lib\http\client.py", line 257, in _read_status
    line = str(self.fp.readline(_MAXLINE + 1), "iso-8859-1")
  File "C:\python3.7.0\lib\socket.py", line 589, in readinto
    return self._sock.recv_into(b)
  File "C:\python3.7.0\lib\ssl.py", line 1049, in recv_into
    return self.read(nbytes, buffer)
  File "C:\python3.7.0\lib\ssl.py", line 908, in read
    return self._sslobj.read(len, buffer)
ConnectionResetError: [WinError 10054] An existing connection was forcibly closed by the remote host