There are two conditions to worry about:

1) The shift count cannot be negative. Since the evaluator uses unsigned
arithmetic throughout, this means checking that the high bit of
the shift count is always off, which is done by converting to int64
and seeing if the result is negative.

2) For right shifts, the value cannot be negative. We don't want a
high bit in the value because right shifting a value depends on the
sign, and for clarity we always want unsigned shifts.

Next step is to build some testing infrastructure for the parser.

Change-Id: I4c46c79989d02c107fc64954403fc18613763f1d
Reviewed-on: https://go-review.googlesource.com/11326Reviewed-by: Russ Cox <rsc@golang.org>