llvm_bugs: A list of bugs I've found in LLVM

As part of my attempt to create a LLVM backend for Chisel/Flo I've
been pretty heavily stressing the multi-word stuff in LLVM.  This
means I'm finding bugs.  I figured it'd be good to try and keep track
of these bugs, as I eventually want to fix them... :)
right-shift by zero loads from the wrong offset

This one is odd: variable right shift by zero produces an incorrect
result.