[InstCombine] When canonicalizing gep indices, prefer zext when possible

If we know that the sign bit of a value being sign extended is zero, we can use a zero extension instead.  This is motivated by the fact that zero extensions are generally cheaper on x86 (and most other architectures?).  We already apply a similar transform in DAGCombine, this just extends that to the IR level.

This comes up when we eagerly canonicalize gep indices to the width of a machine register (i64 on x86_64). To do so, we insert sign extensions (sext) to promote smaller types. 

Differential Revision: http://reviews.llvm.org/D7255

llvm-svn: 229189