[reg scavenger] Fix the isUsed/isAliasUsed functions so as to not report a false

positive.

In this particular case, R6 was being spilled by the register scavenger when it
was in fact dead.  The isUsed function reported R6 as used because the R6_R7
alias was reserved (due to the fact that we've reserved R7 as the FP).  The
solution is to only check if the original register (i.e., R6) isReserved and
not the aliases.  The aliases are only checked to make sure they're available.

The test case is derived from one of the nightly tester benchmarks and is rather
intractable and difficult to reproduce, so I haven't included it.
rdar://12592448

llvm-svn: 168054