Restructure ArgumentPromotion a bit. Instead of just having a single boolean

that says "unconditional loads from this argument are safe", we now keep track
of the safety per set of indices from which loads happen. This prevents
ArgPromotion from promoting loads that aren't really valid. As an added effect,
this will now disregard the the type of the indices passed to a GEP, so
"load GEP %A, i32 1" and "load GEP %A, i64 1" will result in a single argument,
not two.

This fixes PR2598, for which a testcase has been added as well.

llvm-svn: 54159