[PowerPC] Strength reduction of multiply by a constant by shift and add/sub in place

A shift and add/sub sequence combination is faster in place of a multiply by constant. 
Because the cycle or latency of multiply is not huge, we only consider such following
worthy patterns.

```
(mul x, 2^N + 1) => (add (shl x, N), x)
(mul x, -(2^N + 1)) => -(add (shl x, N), x)
(mul x, 2^N - 1) => (sub (shl x, N), x)
(mul x, -(2^N - 1)) => (sub x, (shl x, N))
```

And the cycles or latency is subtarget-dependent so that we need consider the
subtarget to determine to do or not do such transformation. 
Also data type is considered for different cycles or latency to do multiply.

Differential Revision: https://reviews.llvm.org/D58950

llvm-svn: 357233

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