Proposed doc update for Explicit Reg Vars 3/3

[Jeff Law]

On 10/20/2015 10:15 AM, Andrew Haley wrote:
>> But in that case, what do we guarantee.
>>
>> We certainly don't guarantee that those objects will be in their
>> requested register at any point other than at the asm statements.
>
> OK, but this usage did work in the past: that it now doesn't is a
> regression.  GCC was quite useful (unique, really) in that it provided
> a way to write a decent bytecode interpreter in a HLL.  The world has
> lost something significant if this no longer works.
It still works, but again, it's not a guarantee, never has been.

bz21182 has a testcase that's still helped by local register variables.

Jeff