Fix a nasty problem on two-address machines in the following situation:

store EAX -> [ss#0]
[ss#0] += 1
...
use(EAX)

In this case, it is not valid to rewrite this as:


store EAX -> [ss#0]
EAX += 1
store EAX -> [ss#0]  ;;; this would also delete the store above
...
use(EAX)

... because EAX is not a dead at that point.  Keep track of which registers
we are allowed to clobber, and which ones we aren't, and don't clobber the
ones we're not supposed to.  :)

This should resolve the issues on X86 last night.

llvm-svn: 25948