[PowerPC] support ZERO_EXTEND in tryBitPermutation

This patch add a support of ISD::ZERO_EXTEND in PPCDAGToDAGISel::tryBitPermutation to increase the opportunity to use rotate-and-mask by reordering ZEXT and ANDI.
Since tryBitPermutation stops analyzing nodes if it hits a ZEXT node while traversing SDNodes, we want to avoid ZEXT between two nodes that can be folded into a rotate-and-mask instruction.

For example, we allow these nodes

      t9: i32 = add t7, Constant:i32<1>
    t11: i32 = and t9, Constant:i32<255>
  t12: i64 = zero_extend t11
t14: i64 = shl t12, Constant:i64<2>

to be folded into a rotate-and-mask instruction.
Such case often happens in array accesses with logical AND operation in the index, e.g. array[i & 0xFF];

Differential Revision: https://reviews.llvm.org/D37514

llvm-svn: 314655