Clean up some code around the store V, (cast P) -> store (cast V), P

Clean up some code around the store V, (cast P) -> store (cast V), P
transform. Change some variable names so it is clear what is source and
what is dest of the cast. Also, add an assert to ensure that the integer
to integer case is asserting if the bitwidths are different. This prevents
illegal casts from being formed and catches bitwidth bugs sooner.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@33337 91177308-0d34-0410-b5e6-96231b3b80d8