[dev.cc] cmd/asm: check for overflow on multiply and left shift

[dev.cc] cmd/asm: check for overflow on multiply and left shift

The internal size of integers is not part of the definition of the assembler,
so if bits roll out the top it's a portability problem at best.

If you need to use shift to create a mask, use & to restrict the bit count
before shifting. That will make it portable, too.

Change-Id: I24f9a4d2152c3f9f253e22ff75270fe50c18612b
Reviewed-on: https://go-review.googlesource.com/3451
Reviewed-by: Russ Cox <rsc@golang.org>