Fixed GEP visitor in the InstCombine pass.

The current implementation of GEP visitor in InstCombine fails with assertion on Vector GEP with mix of scalar and vector types, like this:

getelementptr double, double* %a, <8 x i32> %i
(It fails to create a "sext" from <8 x i32> to <8 x i64>)

I fixed it and added some tests.

Differential Revision: http://reviews.llvm.org/D14485

llvm-svn: 253162