[SROA] Switch the common variable name for the 'AllocaSlices' class to
'AS'.

Using 'S' as this was a terrible idea. Arguably, 'AS' is not much
better, but it at least follows the idea of using initialisms and
removes active confusion about the AllocaSlices variable and a Slice
variable.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219963 91177308-0d34-0410-b5e6-96231b3b80d8