[PowerPC] Implement BuildSDIVPow2, lower i64 pow2 sdiv using sradi

PPCISelDAGToDAG contained existing code to lower i32 sdiv by a power-of-2 using
srawi/addze, but did not implement the i64 case. DAGCombine now contains a
callback specifically designed for this purpose (BuildSDIVPow2), and part of
the logic has been moved to an implementation of that callback. Doing this
lowering using BuildSDIVPow2 likely does not matter, compared to handling
everything in PPCISelDAGToDAG, for the positive divisor case, but the negative
divisor case, which generates an additional negation, can potentially benefit
from additional folding from DAGCombine. Now, both the i32 and the i64 cases
have been implemented.

Fixes PR20732.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224033 91177308-0d34-0410-b5e6-96231b3b80d8
ARM: convert isTargetIOS checks to isTargetDarwin.

The distinction is mostly useful in the front-end. By the time we get here,
there are very few situations where we actually want different behaviour for
Darwin and IOS (in fact Darwin mostly just exists in a few tests). So this
should reduce any surprising weirdness for anyone using it.

No functional change on anything anyone actually cares about.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224035 91177308-0d34-0410-b5e6-96231b3b80d8
[Hexagon] Renaming classes in preparation for replacement.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224036 91177308-0d34-0410-b5e6-96231b3b80d8