Handle big index in getelementptr instruction

CodeGen incorrectly ignores (assert from APInt) constant index bigger
than 2^64 in getelementptr instruction. This is a test and fix for that.

Patch by Paweł Bylica!

Reviewed By: rnk

Subscribers: majnemer, rnk, mcrosier, resistor, llvm-commits

Differential Revision: http://reviews.llvm.org/D8219

llvm-svn: 231984