[PATCH] [IndVarSimplify] Widen signed loop compare instructions to enable additional optimizations.

[Andrew Trick]

I think the reason removing the trunc works well in practice is that LSR can easily tell that an immediate offset can be folded into the compare. I'm sure it would be possible to make LSR handle the trunc, but fixing Indvars to generate a cleaner loop test is a fine approach.

I'm still curious if this triggers any InstCombines that we were missing before.

http://reviews.llvm.org/D5333