Assembler::PopAndReturn expects an Integral type so convert argc from a
SMI if necessary.

On 64-bit architectures, convert 64-bit immediate pop values into
32-bit values. This is safe since the conversion checks that nothing
was truncated.

Also change CodeStubArguments unit tests to use PopAndReturn rather
than Return.

Change-Id: I91b47d2e81dc0504d185ad59752d638b1c3135a7
Reviewed-on: https://chromium-review.googlesource.com/867052Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50645}