Fix an issue where doasm fails if trying to multiply by a larger
than 32 bit const (doasm: notfound ft=9 tt=14 00008 IMULQ
$34359738369, CX 9 14).  Fix truncation of 64 to 32 bit integer
when generating LEA causing incorrect values to be computed.

Change-Id: I1e65b63cc32ac673a9bb5a297b578b44c2f1ac8f
Reviewed-on: https://go-review.googlesource.com/12678Reviewed-by: Keith Randall <khr@golang.org>