MIPS: Added ASSERTs to BranchShort to detect bad use of 'at' register.

MIPS: Added ASSERTs to BranchShort to detect bad use of 'at' register.

Make sure 'at' and an immediate operand can't be used together in cases
where 'at' would be overwritten by li.

BUG=
TEST=

Committed: http://code.google.com/p/v8/source/detail?r=9072

[Paul Lind, 2011-08-30 07:04:45]

Yang - this patch caught the bad use of 'at' register in our port of your string
slices commit.

[Yang, 2011-08-30 16:15:25]

On 2011/08/30 07:04:45, Paul Lind wrote:
> Yang - this patch caught the bad use of 'at' register in our port of your
string
> slices commit.

LGTM.

[Yang, 2011-08-30 16:15:35]

On 2011/08/30 16:15:25, Yang wrote:
> On 2011/08/30 07:04:45, Paul Lind wrote:
> > Yang - this patch caught the bad use of 'at' register in our port of your
> string
> > slices commit.
> 
> LGTM.

Landed.