[InstCombine] SSE4A constant folding and conversion to shuffles.

This patch improves support for combining the SSE4A EXTRQ(I) and INSERTQ(I) intrinsics:

1 - Converts INSERTQ/EXTRQ calls to INSERTQI/EXTRQI if the 'bit index' and 'length' operands are constant
2 - Converts INSERTQI/EXTRQI calls to shufflevector if the bit index/length are both byte aligned (we can already lower shuffles to INSERTQI/EXTRQI if its useful)
3 - Constant folding support
4 - Add zeroinitializer handling

Differential Revision: http://reviews.llvm.org/D13348

llvm-svn: 250609